Interesting to find this discussion here, maybe this will get some cases solved in the future.
In order to create smaller container names, I shortened the following in mailcow.conf:
COMPOSE_PROJECT_NAME=mailcowdockerized
I shortened it to COMPOSE_PROJECT_NAME=mc
Result: unable to login.
Steps to reproduce:
docker compose down
change the value in mailcow,conf
docker compose up -d
No more authentication possible…
After resetting in the same order, all worked again…