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iRedMail 1.6.2 (github.com/iredmail/iRedMail/archive/refs/tags/1.6.2.tar.gz)
Downloadable installer
RHEL8 (Red Hat Enterprise Linux 8.7)
MySQL
Nginx
Manage mail accounts with iRedAdmin-Pro? Yes
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After installing iRedMail I noticed services iredadmin, mlmmjadmin, iredapd did not start, error was:
dec 23 16:20:32 pushmail.alcad.si iredadmin[1161]: [192.168.5.191] GET /iredadmin 500 104 "-"
dec 28 09:30:40 pushmail.alcad.si iredadmin[1161]: Traceback (most recent call last):
dec 28 09:30:40 pushmail.alcad.si iredadmin[1161]:   File "/opt/www/iRedAdmin-1.8.1/iredadmin.py", line 8, in <module>
dec 28 09:30:40 pushmail.alcad.si iredadmin[1161]:     from libs import iredbase
dec 28 09:30:40 pushmail.alcad.si iredadmin[1161]:   File "/opt/www/iRedAdmin-1.8.1/libs/iredbase.py", line 5, in <module>
dec 28 09:30:40 pushmail.alcad.si iredadmin[1161]:     import web
dec 28 09:30:40 pushmail.alcad.si iredadmin[1161]: ModuleNotFoundError: No module named 'web'
dec 28 09:30:40 pushmail.alcad.si iredadmin[1161]: unable to load app 0 (mountpoint='pushmail.alcad.si|/iredadmin') (callable not found or import error) dec 28 09:30:40 pushmail.alcad.si iredadmin[1161]: --- no python application found, check your startup logs for errors --- dec 28 09:30:40 *****.*****.si iredadmin[1161]: [192.168.5.***] GET /iredadmin 500 104 "-"

I solved the issue with installing missing module:
pip3 install web.py

and then restarted services:
systemctl restart iredadmin mlmmjadmin iredapd

...now it works.