Lets clarify a little
.
sql_auth.php perform like a proxy. It take from ispcp.conf BASE_SERVER_IP, (previvios was BASE_SERVER_VHOST), username and password is taken from database (password is decrypted) then using POST method with fsockopen sent to BASE_SERVER_IP/pma/ login data. If BASE_SERVER_IP is wrong (previous BASE_SERVER_VHOST, I droped this because it need to convert first into IP so I took BASE_SERVER_IP instead) or it cannot connect to it you will get an Error while authenticating!. If everything was ok server will return some cookies and a redirect. This are passed via setcookie and header functions to browser and you get are logged in. If something go wrong you also get a nice Error while authenticating!. If you alter some way how pma is default handled, you get errors (like you did with a port).
I did explain this to eliminate every "custom errors". Anyway I`ll check it today / tonight and see what else can make it to fail. Maybe some characters from password and username? Can you provide some examples that do fail.
Thank you for helping me with this.
That ticket seems to be another problem.