Can't log in - always times out with a 503. Was working fine before

My PHPList has been working fine for many months (thanks), but today all logins timeout. Login is left hanging after entering the username and password until it gives me an error 503 after more than a minute. The login, signup, and about pages displays ok.

I’ve not made changes in recent weeks to the PHPList or to the related hosting environment.

I’m a bit of a loss on how to fix since no errors are returned. I can’t find anything logged. I can view the event log via PHPMyadmin, but there’s not much to see apart from CURLs to send transaction emails timeout (they never used to).

CPanel shows no problem with server load. The WordPress site on the server is running fine. Are there other logs created? How can I debug?

Any pointers would be much appreciated. Thanks

PHPlist version 3.6.10 (i think). PHP Version 7.4.33

I spend the whole day trying to work out what’s broken, and then it just starts to work again. I guess I should be pleased. I hope it doesn’t happen again. I’ll update if I workout what the problem was.