It seems that I am also having issues logging into phpmyadmin as well... I don't use it much as I usually use Navicat Mysql to manage my databases.
phpMyAdmin is doing the exact same thing as ispconfig3. Does not login and no error message.
I repaired the dbispconfig table in Navicat Mysql as well as the mysql table, but am still having the same issue. very strange, going to lookup this problem for phpMyAdmin and see if that can help me solve both problems.