I did not yet noticed when this problem actually starts, but after I've been online for a while and I press Page Up or Page Down to scroll my chatwindow, I get an error message.
The messages are as follow:
Interface\FrameXML\ChatFrame.lua:2508:attempt to call method 'PageDown' (a nil value)
Interface\FrameXML\ChatFrame.lua:2504:attempt to call method 'PageUp' (a nil value)
I scrolled 2-3 pages back to see if anyone else had this, but I couldn't find anything so posting it.