Not trying to thread steal, but I am having the same issue. I was on 1.17,x upgraded to 1.17.4, and then upgraded to 1.18.
Anytime we would try to login on the public side, post a reply, or update settings we would get a 500 error. We have reviewed logs and aren't seeing anything that would lead us directly to an issue.
After the upgrade to 1.18, we are unable to remove oath plugin. Oauth2 Client (1) (defunct — missing). If we click delete it gives 500 error. We deleted all emails and setup a new temporary email just to be sure. Still seeing the same issue.