@joseph thanks!
No, no errors inside of docker image logs. /home/yellowtent/platformdata/logs/mail/*.log - from what I can see, that is the same logs I can see inside of Docker - so I would say there are no (error) logs - from all of the perspective, the server looks perfectly normal... except that it doesn't accept user's messages...
I would hope that you bring updated Dovecot to Cloudron 9 and it will self-resolve, but better will just do a restart for the time being...