I am having the same issue, cannot log in to phpMyAdmin.
Web Server Enabled - CHECK (default ports)
MySQL Server Enabled - CHECK (I even re-initialized the database and reset root password to default)
phpMyAdmin Updated to most recent version - CHECK
QNAP Updated to most recent version - CHECK
Errors I'm getting:
#2013 Cannot log in to the MySQL server
Login without a password is forbidden by configuration (see AllowNoPassword)
Weird thing is that the user and password lines of the config.inc.php file seem to be commented out. Also, there is no line to set the AllowNoPassword feature. So, this is making me think that maybe I'm in the wrong config file. I tried uncommenting the user/pass lines and that didn't fix it either.
I think I'm going to try to uninstall and reinstall the whole thing next. Hopefully that fixes whatever bug I've got. Any other ideas?