I use windowed but with frame. I use Dermott's Icon pack and I have inside Pinco's UI set the mini-texture Dermott's Leather.
I has just started doing this today. I've played for hours and hours yesterday and the days leading up to the big IDOC day. I'm positive it started today.
from your screenshot is clear that you are using the dermott copper texture pack, that is not working for the latest version.
Try this:
1) uninstall the UI from the UI patcher
2) clear all the files from the UserInterface folder (leave Default.zip)
3) delete waypoints.xml from UserData
4) backup your UserData folder
5) empty the UserData folder.
6) reinstall the UI.
(DO NOT INSTALL ANY TEXTURE PACK)
7) try if the problem persist (don't bother to manually restore your settings, just try for the problem)
8) if the problem is not showing, close the game and try to restore the UserData folder (your backup).
9) try again with your settings, so we can see again if you had this problem.
(In this case we can understand if is one of your settings to cause it).
then let me know