How to fix error 503 (asp.net core)?

Question

Nginx

After publishing the IP in production, I began to receive an error 503. Sometimes even on the authorization page.
Linux server, nginx reverse proxy, after kestrel, asp.net core MVC + dapper + postgresql
Almost all functions are asynchronous.
I also turned on static caching, I still get 503 exactly
. (I forgot to add, signalR is also used)
What am I doing wrong? Can anyone come across?

There are about 1200 stable users.
According to Google Analytics, there are about 35-40 thousand active sessions.

Answer 1

503 is an internal server error, where an exception is thrown, turn on the logs and look at them