[spiel2001]

For those of you who are experiencing this problem...

Can you please edit [ Interface > AddOns > nUI > Bars > nUI_Button.lua ] and go to line 300 where you see the following code and add the line that's in blue... then log in and when you get the error message, look to see what the last line was that was printed and tell me what it says the "id" equals?

 
               action   = ActionButton_CalculateAction( self );
                type, id = GetActionInfo( action );

                print( "id = ["..(id or "nil").."]" );

                if id 
                and id > 0
                and nUI.lastSpell
                and type == BOOKTYPE_SPELL 
                and id <= nUI.lastSpell
                then

Once you've found out what the last ID was that threw the error, you can remove the blue line again so it doesn't spam your chat frame anymore.