==== REQUIRED BASIC INFO OF YOUR IREDMAIL SERVER ====
- iRedMail version (check /etc/iredmail-release):
- Deployed with iRedMail Easy or the downloadable installer?
- Linux/BSD distribution name and version:
- Store mail accounts in which backend (LDAP/MySQL/PGSQL):
- Web server (Apache or Nginx):
- Manage mail accounts with iRedAdmin-Pro?
- [IMPORTANT] Related original log or error message is required if you're experiencing an issue.
====
I have an older iredmail installation that uses iRedApd-2.3. Due to some issues, I needed to switch to mysql from 5.4 (we have a site database server all use). I was able to make everything fly, but now I see that when iredapd conencts to mysql, it bombs out with Error 255 and logs the following message:

ERROR Unexpected error: 255. Fallback to default action: DUNNO

From the command line of this host, I am able to connect and log in with the exact name/password that is listed in settings.py. I think the problem is some kind of library incompatibility with mysql 8 because the server doesn't even report the login attempt and under strace really all I see is the initial tcp connect, the server sends the mysql greeting string (binary garbage with the string "native_password" at the end), and then the iredapd just barfs and closes the connection.
startup of iredapd looks normal - no errors or anything noted:

2021-12-23 10:01:00 INFO Starting iRedAPD (version: 2.3, backend: mysql), listening on 127.0.0.1:7777.
2021-12-23 10:01:00 INFO Log rotate type: time, interval: W6, backup copies: 12.
2021-12-23 10:01:00 INFO Loading plugin (priority: 100): reject_null_sender
2021-12-23 10:01:00 INFO Loading plugin (priority: 99): wblist_rdns
2021-12-23 10:01:00 INFO Loading plugin (priority: 60): throttle
2021-12-23 10:01:00 INFO Loading plugin (priority: 50): sql_alias_access_policy
2021-12-23 10:01:00 INFO Loading plugin (priority: 40): amavisd_wblist

I am kinda thinking this is relatively easy - some flag or option to set in the python connector, but I am not smart enough with python to figure this out. I have toyed around with different versions (I am on old iredmail and upgrade is not possible at this time) and am just not seeing an easy solution.
Thank you.