Besides yourself I see 2 Administrators on your forum.   Click your Members tab - it clearly shows them there.   Your Staff List also shows then as Administrators.

I'm also gonna repeat (again) do not make a bunch of Admins on your forum.   You have had repeated problems with doing this, it has had to be fixed for you many times already.     We are no longer fixing forums that are lost due to inner forum squabbles/too many admins and one takes over.     Might want to take this as a piece of advice - STOP making people Admin on your forum.

Your forum was not "hacked" and you do not have a "hacker"     You gave Administrative access to others on your forum.  One of them changed Membergroup Permissions for other Groups on your forum.    You as ADmin can fix that.    First take Admin access AWAY from everyone on your forum.    Only YOU should have it.   Then put your forum into Maintenance Mode while you fix the rest of it.   Go to Admin > Permissions.   Modify each of your Membergroups.   Check to see how the permissions are set for every group on your forum.   Make sure your members are in only ONE group and are not set to Admin as a secondary Group.     You are going ot have to go thru all of your settings to get this all fixed.  This is not something we do for you.      sergiem , like I said, you really need to stop giving people Admin access on your forum.

Because you made people, the administrators that are willing to mess up the forum whatever the cause. Like simply sibyl said, it was not hacked, you gave away too much administration privileges to members YOU DO NOT KNOW nor TRUST in this matter and they messed up the forum settings.

So technically, it's YOUR FAULT that you put them in the "Administration" staff in the first place.

I suggest like simply sibyl said, do not GIVE OUT any administration privileges to people that you do NOT know, unless you're taking a BIG risk. Give it to someone LOYAL that you KNOW, that WILL NOT mess up the forum.

That's my advice for you man and I wish you luck rebuilding the forum. Remember that. Thanks!