Got a message on the inventory window this morning and removed the eq2ui_inventory_inventory.xml window from my UI folder. After that there was no more message.
I also see where these two files should be removed as well (thanks Chaaskul):
FYI, you don't have to copy files from the default folder - just delete them from your custom UI folder. Anytime the files are missing from a custom UI folder, it uses the ones from the DEFAULT folder anyway.