I use FreeBSD 11.3-PRERELEASE #3 r349549, cyrus-imap 2.5. When linking with Berkeley DB I have intermittently gotten database errors, even though I never specified Berkeley DB in the configuration file. I wasn't quite sure why I was getting Berkeley DB-like files in the db directory (i.e. __db.001, __db.002, __db.003 which looked like Berkeley DB files).
Lately I started getting the following errors from the different daemons:
squatter: DBERROR db5: pthread suspend failed: Invalid argument
I found that I'd configured Berkeley DB support and that all the Cyrus binaries were linked with Berkeley DB 5.3. I removed the Berkeley DB option, reinstalled, and rebuilt the /var/imap directory and now things work OK, with no Berkeley DB files appearing.
Suggestion: remove the Berkeley DB option or at least note that Cyrus Imap will still use Berkeley DB for some things even if you don't configure it in the config file. Also the Berkeley DB support seems to have bit-rotted.