Got previously issues with PostgreSQL 13 update, Restore problem: relation "theme_fields" does not exist and How to backup and restore a whole /var/discourse app folder? - #15 by adrelanos. Now very happy for the service of being provided with the slower stable branch. 
I guess you’re on version 2.7. Will be waiting until 2.7 is migrated to the stable branch and report back.
are you able to provide any additional reproduction steps?
min post length is set to 2.
delete rejected email after days is still default 90.
Replying “OK” to a message of myself is now functional.
Replaying “OK” got another user’s message I now get.
We’re sorry, but your email message to [“discoursereplies+redacted@redacted.redacted”] (titled Re: [redacted] [redacted] redacted) didn’t work.
We only accept replies to original notifications for 90 days. Please visit the topic to continue the conversation.
But the message was just 5 minutes or so old.
Replying by e-mail to post by other user with text “This is a longer test reply by e-mail. Please ignore.” nothing happens, nothing shows up in log either. No rejection e-mail received either.
Is it possible that the message you received was valid somehow?
What do you mean by was valid?