I got a new monitor today. At first the problem didn't exist, but now I have unusual boxes appearing around all action bars (See screenshot).
I've removed ALL add ons completely, but the problem still remains.
Try in the /interface folder, and see if there are any folders there besides Addons.
Typically there is where you might put artwork replacements, which given the presence of those graphics without any addons would be a good candidate.
Blizzard uses a very tiny default actionbutton "normaltexture". That is why they have to stretch the normaltexture setpoint way bigger than the actual button to make it fit.
There are actionbutton mods (like my styler) who use full-button-width normaltextures. Seems like you had one of those installed. Those work if you have an actionbutton mod installed that fix the normaltexture setpoint. Once that actionbar mod is removed it will look wierd since the full-button-width normaltexture becomes stretched.