I created a new usergroup with same permission as registered usergroup.

When I a user in the new usergroup logs in it gets the logging in notice but then goes to a message saying

Testuser, you do not have permission to access this page. This could be due to one of several reasons:

Your user account may not have sufficient privileges to access this page. Are you trying to edit someone else's post, access administrative features or some other privileged system?
If you are trying to post, the administrator may have disabled your account, or it may be awaiting activation.
And there is a warning at the top of the page that says

Warning: array_merge() [function.array-merge]: Argument #1 is not an array in /home/public_html/includes/functions.php on line 727
I have tried disabling plugins and reverting templates but nothing seems to work.

My php version is 5.2.9.

Any suggestions or ideas why this might be happening?

Thank you

EDIT: FIGURED OUT WHAT WAS CAUSING BY DISABLING HOOKS.