You probably have your ACP set to different language than english. This bug was fixed in 3.2.3 https://github.com/NodeBB/NodeBB/issues/11802. You can upgrade to 3.2.3 or 3.6.1 to resolve it.
@baris no logs in server terminal (./nodebb log). error loading 500 Error: Cannot find module ./500 admin.min.js?v=??? in browser console. And 500.tpl is rendered with no-connection error message.
Open up logs/output.log after the error happens ./nodebb log only shows the last few lines. Maybe the error is in there. Were you using a custom emailer?