Seems a bug of Python module 'py-ldap'. May i know the version number of package 'net/py-ldap' running on your server?
Did you update the ports tree before installing iRedMail?

Hi.

I had the same problem on 11.1.

Downgrade to py27-ldap-2.4.45, works.

I'm having the same issue on 11.1, with a fresh ports tree and bone-stock install.  I'm on py-ldap 2.5.2.  I'll tray a portdowngrade and see what comes of it.

iRedMail version (check /etc/iredmail-release): 0.9.8-beta1 & 0.9.7
- Linux/BSD distribution name and version: FreeBSD 11.1 and Ubuntu 16.04-17.10
- Store mail accounts in which backend (LDAP/MySQL/PGSQL): LDAP
- Web server (Apache or Nginx): Apache or ngnix
- Manage mail accounts with iRedAdmin-Pro? no
- [IMPORTANT] Related original log or error message is required if you're experiencing an issue.

This is on iredadmin 0.9.8-beta1, but I had a similar issue on 0.9.7, using both apache and ngnix, and with similar results on ubuntu.

roundcube works and email can get delivered. I can login through roundcube and I can also login using an MUA to the default postmaster, but I definitely see the same errors in my /var/log/uwsgi-iredadmin.log file

EDIT: Perhaps this change in November may have borked things?:

r455210 | mat | 2017-11-30 15:50:30 +0000 (Thu, 30 Nov 2017) | 35 lines

Convert Python ports to FLAVORS.

  Ports using USE_PYTHON=distutils are now flavored.  They will
  automatically get flavors (py27, py34, py35, py36) depending on what
  versions they support.

  There is also a USE_PYTHON=flavors for ports that do not use distutils
  but need FLAVORS to be set.  A USE_PYTHON=noflavors can be set if
  using distutils but flavors are not wanted.

  A new USE_PYTHON=optsuffix that will add PYTHON_PKGNAMESUFFIX has been
  added to cope with Python ports that did not have the Python
  PKGNAMEPREFIX but are flavored.

  USES=python now also exports a PY_FLAVOR variable that contains the
  current python flavor.  It can be used in dependency lines when the
  port itself is not python flavored.  For example, deskutils/calibre.

  By default, all the flavors are generated.  To only generate flavors
  for the versions in PYTHON2_DEFAULT and PYTHON3_DEFAULT, define
  BUILD_DEFAULT_PYTHON_FLAVORS in your make.conf.

  In all the ports with Python dependencies, the *_DEPENDS entries MUST
  end with the flavor so that the framework knows which to build/use.
  This is done by appending '@${PY_FLAVOR}' after the origin (or
  @${FLAVOR} if in a Python module with Python flavors, as the content
  will be the same).  For example:

    RUN_DEPENDS= ${PYTHON_PKGNAMEPREFIX}six>0:devel/py-six@${PY_FLAVOR}

PR:             223071
Reviewed by:    portmgr, python
Sponsored by:   Absolight
Differential Revision:  https://reviews.freebsd.org/D12464

 test.mydomain.com [pid: 3124|app: 0|req: 3/3] xx.xx.xx.xx () {52 vars in 884 bytes} [Tue Feb 13 23:14:26 2018] GET /iredadmin => generated 13 bytes in 30 msecs (HTTP/1.1 303) 3 headers in 257 bytes (2 switches on core 0)
Traceback (most recent call last):
  File "/usr/local/lib/python2.7/site-packages/web/application.py", line 239, in process
    return self.handle()
  File "/usr/local/lib/python2.7/site-packages/web/application.py", line 230, in handle
    return self._delegate(fn, self.fvars, args)
  File "/usr/local/lib/python2.7/site-packages/web/application.py", line 458, in _delegate
    mod = __import__(mod, None, None, [''])
  File "/usr/local/www/iRedAdmin-0.9/controllers/ldap/basic.py", line 11, in <module>
    from libs.ldaplib import auth, decorators, admin as adminlib, ldaputils
  File "/usr/local/www/iRedAdmin-0.9/libs/ldaplib/admin.py", line 7, in <module>
    from libs.ldaplib import core, attrs, ldaputils, iredldif, deltree, connUtils, decorators
  File "/usr/local/www/iRedAdmin-0.9/libs/ldaplib/deltree.py", line 5, in <module>
    class DeleteLeafs(ldap.async.AsyncSearchHandler):
  File "/usr/local/www/iRedAdmin-0.9/libs/ldaplib/deltree.py", line 11, in DeleteLeafs
    _entryResultTypes = ldap.async._entryResultTypes
AttributeError: 'module' object has no attribute '_entryResultTypes' 

I'm trying to reproduce this issue.