Hi all, thanks for your feedback here. Last Tuesday, we published an update to the ESO patcher in preparation for Update 31. This prep work seems to be triggering the UAC popup as update 31 is not available yet.
UAC updates can occur on Windows when a missing game dependency, like update 31, is not live yet. Most windows users have setting to require admin approval for installations.
Right now, if you don't approve the UAC login, the game will work as of now since this is just a preparatory step for a later game update.
Let us know if you have any further issues on this and we'll keep an eye on feedback. Thanks!