After upgrading a plesk install yesterday. From 10.1 to 10.3.1 the admin panel started redirecting me from myserver.com:8443 to myserver.com:8443/relay. Which resulted in a 404 error with no plesk panel. During the upgrade plesk must have activated the sso component which stopped the webinterface from working. To fix this issue do the following:

/usr/local/psa/bin/sso --disable