This is what the log says in the moment of crashing:
dovecot: lmtp(53852): Panic: Module context mail_crypt_user_module missing
*** backtrace *** (see: https://pastebin.com/YCiFtxmy)

It shows mailbox_free() in the backtrace, so it's probably crashing in mail_crypt_mailbox_close()'s MAIL_CRYPT_CONTEXT(box). But looking at the code, I can't see how that could be possible. Maybe there is some kind of memory corruption or something.

dovecot: lmtp(53852): Fatal: master: service(lmtp): child 53852 killed with signal 6 (core dumps disabled - https://dovecot.org/bugreport.html#coredumps)

Could you get core dumps enabled? See the link. And from the core dump send at least:

bt full
fr 6 (or maybe 5 or 7, whichever makes the following command work)
p *box

Also please send your doveconf -n output.

Core dump: https://pastebin.com/g4VkMXsT (hope this is everything - I’m not that familiar with core dumps…)