Error 500 on WP site, 2 other WP sites on same ispconfig runs fine

Hi!
I have a site that gives me Error 500 when I try to login to WP.
I have 2 other sites with WP on same ISPconfig server, that login to fine.
I do get the login prompt in WP (http://www.domain.tld/wp-login.php?redirect_to=http://www.domain.tld/wp-admin/&reauth=1
But clicking OK, when username/password is entered gives the Errror 500.
There is nothing to see in either error.log or access.log

How do I troubleshoot ? Any help appreciated...

 

Forgot to say it's running with NGINX...

 

OK, since it's only one of the sites having problems. I deactivated all plug-ins on that site in the database, with the following query in SQL:

Code:

UPDATE wp_options SET option_value = 'a:0:{}' WHERE option_name = 'active_plugins';