If mysql 5.5.x was used and the problem occurred in bbpress forum installed within buddypress,
1) delete buddypress.
2) It might be useful to check if the tables for bbpress were deleted. Check also there is no bb-config.php in the web root folder.
3) reinstall buddypress
4) change ‘TYPE = MYISAM’ to ‘ENGINE = MYISAM’ in /wp-content/plugins/buddypress/bp-forums/bbpress/bb-admin/includes/defaults.bb-schema.php before creating groups and using forums.
5) create groups and test forums.