The issues related to logging in to the European PC/Mac megaserver have been resolved at this time. If you continue to experience difficulties at login, please restart your client. Thank you for your patience!
I believe it's a reference to the underline graphic. If I understand it correctly the ESO UI (used in the various addons) can display various graphics, but to display them the addon needs to know which image file to load.
PC(Steam) / EU / play from Melbourne, Australia / avg ping 390