With "internal server error", i expect some error log in /var/log/message.
Could you try to reproduce this error again and check all log files under /var/log/?

I went directly from 3.6 to 3.9. I'm going to try 3.7 and 3.8 to see if they work...

Upgrade to 3.7 works fine.
Upgrade to 3.8 gives internal server error, same as 3.9

I spoke too soon - 3.7 allows access to the interface, but internal server error when accessing a user.

Update - this may be my fault - I made some alterations to allow for new LDAP attributes, and I likely broke it myself. I'll update this message if that is the case.

It wasn't that - I redid the update to 3.7 and it is working fine, no more errors. 3.8 and 3.9 are not. I'm going to continue trying, and go through logs to see if I can figure it out - which log file would generally log the internal server error? I'm not seeing any errors related to this in any log files.

Thanks.
I set MAIL_ERROR_TO_WEBMASTER = True, and here are the results:

Error after upgrading to 3.8:
email subject line: bug: <class 'socket.error'>: [Errno 13] Permission denied (/)

Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/web/application.py", line 239, in process
    return self.handle()
  File "/usr/lib/python2.7/site-packages/web/application.py", line 230, in handle
    return self._delegate(fn, self.fvars, args)
  File "/usr/lib/python2.7/site-packages/web/application.py", line 416, in _delegate
    mod = __import__(mod, None, None, [''])
  File "/var/www/iRedAdmin-Pro-LDAP-3.8/controllers/ldap/basic.py", line 9, in <module>
    from libs import iredutils, sysinfo, form_utils
  File "/var/www/iRedAdmin-Pro-LDAP-3.8/libs/sysinfo.py", line 13, in <module>
    from libs.logger import log_traceback
  File "/var/www/iRedAdmin-Pro-LDAP-3.8/libs/logger.py", line 30, in <module>
    _handler = SysLogHandler(address=_server, facility=settings.SYSLOG_FACILITY)
  File "/usr/lib64/python2.7/logging/handlers.py", line 761, in __init__
    self._connect_unixsocket(address)
  File "/usr/lib64/python2.7/logging/handlers.py", line 789, in _connect_unixsocket
    self.socket.connect(address)
  File "/usr/lib64/python2.7/socket.py", line 224, in meth
    return getattr(self._sock,name)(*args)
error: [Errno 13] Permission denied

Error after upgrading to 3.9:
email subject line: bug: <class 'socket.error'>: [Errno 13] Permission denied (/)

Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/web/application.py", line 239, in process
    return self.handle()
  File "/usr/lib/python2.7/site-packages/web/application.py", line 230, in handle
    return self._delegate(fn, self.fvars, args)
  File "/usr/lib/python2.7/site-packages/web/application.py", line 416, in _delegate
    mod = __import__(mod, None, None, [''])
  File "/var/www/iRedAdmin-Pro-LDAP-3.9/controllers/ldap/basic.py", line 9, in <module>
    from libs import iredutils, sysinfo, form_utils
  File "/var/www/iRedAdmin-Pro-LDAP-3.9/libs/sysinfo.py", line 13, in <module>
    from libs.logger import log_traceback
  File "/var/www/iRedAdmin-Pro-LDAP-3.9/libs/logger.py", line 30, in <module>
    _handler = SysLogHandler(address=_server, facility=settings.SYSLOG_FACILITY)
  File "/usr/lib64/python2.7/logging/handlers.py", line 761, in __init__
    self._connect_unixsocket(address)
  File "/usr/lib64/python2.7/logging/handlers.py", line 789, in _connect_unixsocket
    self.socket.connect(address)
  File "/usr/lib64/python2.7/socket.py", line 224, in meth
    return getattr(self._sock,name)(*args)
error: [Errno 13] Permission denied

symlink for iredadmin set back to iRedAdmin-Pro-LDAP-3.7 and everything works properly

the only entries in the other logs are the same as before:
/var/log/messages:
Aug 15 05:26:37 mail systemd[1]: Stopping iRedAdmin daemon service...
Aug 15 05:26:38 mail systemd[1]: Stopped iRedAdmin daemon service.
Aug 15 05:26:38 mail systemd[1]: Starting iRedAdmin daemon service...
Aug 15 05:26:38 mail systemd[1]: Started iRedAdmin daemon service.
Aug 15 05:26:38 mail uwsgi[9797]: [uWSGI] getting INI configuration from /var/www/iredadmin/rc_scripts/uwsgi/rhel.ini

/var/log/nginx/access.log:
192.168.104.68 - - [15/Aug/2019:05:24:05 -0400] "GET /admin HTTP/1.1" 500 32 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:68.0) Gecko/20100101 Firefox/68.0"
192.168.104.68 - - [15/Aug/2019:05:30:13 -0400] "GET /admin HTTP/1.1" 500 32 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:68.0) Gecko/20100101 Firefox/68.0"

Does any of that make sense?

FYI, all updates have gone without a problem since the initial install of iRedAdmin-Pro-LDAP-3.1 up to 3.7. upgrades to 3.8 and 3.9 result in the internal server error.

No.
As I had indicated, I upgraded to 3.7 from 3.6 without any issues, but it's the 3.8 and 3.9 installs that are giving the error.

lsof -i :7791
COMMAND  PID      USER   FD   TYPE DEVICE SIZE/OFF NODE NAME
uwsgi   2133 iredadmin    6u  IPv4  25910      0t0  TCP localhost:7791 (LISTEN)
uwsgi   2352 iredadmin    6u  IPv4  25910      0t0  TCP localhost:7791 (LISTEN)
uwsgi   2353 iredadmin    6u  IPv4  25910      0t0  TCP localhost:7791 (LISTEN)
uwsgi   2358 iredadmin    6u  IPv4  25910      0t0  TCP localhost:7791 (LISTEN)
uwsgi   2360 iredadmin    6u  IPv4  25910      0t0  TCP localhost:7791 (LISTEN)
uwsgi   2362 iredadmin    6u  IPv4  25910      0t0  TCP localhost:7791 (LISTEN)

3.7 working fine
then:
service iredadmin stop
- nothing listening on 7791:
change symlink for iredadmin to either 3.8 or 3.9 and stop then start iredadmin the internal server error is back. Rebooted server, same results.
Set symlink back to 3.7, everything is fine again.

GET https://server.example.net/admin/

Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/web/application.py", line 239, in process
    return self.handle()
  File "/usr/lib/python2.7/site-packages/web/application.py", line 230, in handle
    return self._delegate(fn, self.fvars, args)
  File "/usr/lib/python2.7/site-packages/web/application.py", line 416, in _delegate
    mod = __import__(mod, None, None, [''])
  File "/var/www/iRedAdmin-Pro-LDAP-3.8/controllers/ldap/basic.py", line 9, in <module>
    from libs import iredutils, sysinfo, form_utils
  File "/var/www/iRedAdmin-Pro-LDAP-3.8/libs/sysinfo.py", line 13, in <module>
    from libs.logger import log_traceback
  File "/var/www/iRedAdmin-Pro-LDAP-3.8/libs/logger.py", line 30, in <module>
    _handler = SysLogHandler(address=_server, facility=settings.SYSLOG_FACILITY)
  File "/usr/lib64/python2.7/logging/handlers.py", line 761, in __init__
    self._connect_unixsocket(address)
  File "/usr/lib64/python2.7/logging/handlers.py", line 789, in _connect_unixsocket
    self.socket.connect(address)
  File "/usr/lib64/python2.7/socket.py", line 224, in meth
    return getattr(self._sock,name)(*args)
error: [Errno 13] Permission denied

Same result.
added that line to settings.py in /var/www/iRedAdmin-Pro-LDAP-3.9
settings.py:SYSLOG_SERVER = '/dev/log'

I also tried: SYSLOG_SERVER = '/run/systemd/journal/syslog' as per the suggestion in libs/default_settings.py

ls -l /dev/log
srw-rw---- 1 root mysyslog 0 Aug 15 07:59 /dev/log

Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/web/application.py", line 239, in process
    return self.handle()
  File "/usr/lib/python2.7/site-packages/web/application.py", line 230, in handle
    return self._delegate(fn, self.fvars, args)
  File "/usr/lib/python2.7/site-packages/web/application.py", line 416, in _delegate
    mod = __import__(mod, None, None, [''])
  File "/var/www/iRedAdmin-Pro-LDAP-3.9/controllers/ldap/basic.py", line 9, in <module>
    from libs import iredutils, sysinfo, form_utils
  File "/var/www/iRedAdmin-Pro-LDAP-3.9/libs/sysinfo.py", line 13, in <module>
    from libs.logger import log_traceback
  File "/var/www/iRedAdmin-Pro-LDAP-3.9/libs/logger.py", line 30, in <module>
    _handler = SysLogHandler(address=_server, facility=settings.SYSLOG_FACILITY)
  File "/usr/lib64/python2.7/logging/handlers.py", line 761, in __init__
    self._connect_unixsocket(address)
  File "/usr/lib64/python2.7/logging/handlers.py", line 789, in _connect_unixsocket
    self.socket.connect(address)
  File "/usr/lib64/python2.7/socket.py", line 224, in meth
    return getattr(self._sock,name)(*args)
error: [Errno 13] Permission denied

OK , I tried changing permission on the /dev/log socket, but it seems permissions can't be changed with chmod, so I re-created the socket (systemctl restart systemd-journald.socket) which gave rw permissions to all:
srw-rw-rw- 1 root root           0 Aug 27 07:48 log

set iredadmin to 3.9, got the internal server error, and the socket changed itself back to:
srw-rw---- 1 root mysyslog        0 Aug 27 07:51 log

I tried setting the /dev directory to 777 with the same result.

Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/web/application.py", line 239, in process
    return self.handle()
  File "/usr/lib/python2.7/site-packages/web/application.py", line 230, in handle
    return self._delegate(fn, self.fvars, args)
  File "/usr/lib/python2.7/site-packages/web/application.py", line 416, in _delegate
    mod = __import__(mod, None, None, [''])
  File "/var/www/iRedAdmin-Pro-LDAP-3.9/controllers/ldap/basic.py", line 9, in <module>
    from libs import iredutils, sysinfo, form_utils
  File "/var/www/iRedAdmin-Pro-LDAP-3.9/libs/sysinfo.py", line 13, in <module>
    from libs.logger import log_traceback
  File "/var/www/iRedAdmin-Pro-LDAP-3.9/libs/logger.py", line 30, in <module>
    _handler = SysLogHandler(address=_server, facility=settings.SYSLOG_FACILITY)
  File "/usr/lib64/python2.7/logging/handlers.py", line 761, in __init__
    self._connect_unixsocket(address)
  File "/usr/lib64/python2.7/logging/handlers.py", line 789, in _connect_unixsocket
    self.socket.connect(address)
  File "/usr/lib64/python2.7/socket.py", line 224, in meth
    return getattr(self._sock,name)(*args)
error: [Errno 13] Permission denied