I recently decided to start WoW again to play a Bloodelf Paladin, so I downloaded some UI mods, but when I joined a raid group a frame appeared and I have no idea to which addon it belongs..I can't move it with any Shift/Alt/Ctrl + mouse button combination and the tooltip isn't very helpful either.
Not sure on that one. Looks like Grid but it is not Grid. Post a list of AddOns you use. Alternatively disable all your AddOns, then enable one, log in, check if the frame is there. If it is not, go and enable the next AddOn. Rinse and Repeat until you find the culprit.