In your screenshot, data saver is active (the ring with the plus in the upper right corner).
Have you checked if the Android 14 update has accidentally modified your data saver settings?
I had to disable and re-enable data saver for a few apps after the Android 14 apps (However, not K-9 which worked fine for me).
And just an idea: Was adaptive connectivity enabled through the update? Some apps might not use the WiFi connection as my mobile connection can be deemed “better” or “more stable”. If you are then on a mobile network without DS, an IPv6 connection to a POP3 server expecting IPv4 may fail.
The server name K-9 Mail is using does not match the names in the server certificate. You should have the option to manually accept the invalid server certificate, but that appears not to be working in your case. That bug is already fixed in the current beta version. That version will also automatically detect the proper server name to use.
For now just use pop.server311.com for the incoming server name and smtp.server311.com for the outgoing server name.
Side note: Try to avoid using POP3 with K-9 Mail. If K-9 Mail is the only place you keep your messages, they are trapped there because currently there’s no way to export those messages or copy them to a different account. If you use K-9 Mail to only download a copy, but leave messages on the server to be picked up by another client, you might as well use IMAP with K-9 Mail. You can still use POP3 to download and remove messages from the server using another client.