Okay, I’m fairly sure you’re seeing this issue:
https://github.com/metabase/metabase/issues/11048
I have no idea how to reproduce the cause of this, so I’m not sure what the fix is.
How long is your encryption key (characters)? Do NOT post your full key!
The errors looks similar to your original syslog, so I’m not sure what’s going on, since it starts now. I don’t think it’s a cache.
When this problem is solved, then I would highly recommend that you use a reverse-proxy in front of Metabase. This can be used for many things - even simple free SSL certificates via Let’s Encrypt. You can use any webserver like Nginx, Caddy or Apache. And you can find many great tutorials on https://www.digitalocean.com/community/tutorials?q=reverse+proxy