I've encountered weird issue with Joomla and MySQL. Joomla was working just fine on the outside, but not on the inside. For example it was not possible to create new articles because save action resulted in something like:

1044 Access denied for 'user'@'%' to database 'dbname'
/web/htdocs1/xxx/home/www/libraries/joomla/database/driver/mysqli.php:665

Turning on all kind of sorts of debugging turned out to be useless for this one. Searching various forums and blogs seemed to be useless as well. Since I knew which file and roughly which line, I've modified `mysqli.php` and added debugging print-outs which I deemed to be useful:

1044 LOCK TABLES `xxx` WRITE
/web/htdocs1/xxx/home/www/libraries/joomla/database/driver/mysqli.php:665

Not exactly the best and complete information, however it was enough for support ticket to web hosting provider:

GRANT LOCK TABLES ON `dbname`.* TO 'user'@'%';

All is good now and Joomla seems to be fully operational. Yay!