Re: dovecot + sieve: message recovery possible after rule pointed to invalid mailbox?

error: msgid=433788c81ea044c4ae49cb39a6f673f3@ALB-DC-EXC08.CLPL.INTERN: failed to store into mailbox 'ABC': Internal error occurred. Refer to server log for more information. [2022-04-28 07:41:10].

Which "server log" is meant in this error message?

This would be your mail logs, usually somewhere inside of /var/log/ depending on your distro. Sometimes in /var/log/messages or /var/log/maillog or other.