So it appears ... but ...
It's usually recommended to upgrade to the highest in the current series then upgrade to X.
Moodle 2.6.11+ was the highest available in that series.
Min. Requirements: PHP 5.3.3, MySQL 5.1.33
Moodle 3.0.5+
Min. Requirements: PHP 5.4.4, MySQL 5.5.31
You've not mentioned version of MySQL.
If you upgrade the MySQL make sure you run mysql_upgrade as super user.
Also, is the DB server localhost or is it a remotely hosted DB server?
'spirit of sharing', Ken