I'm getting this too. I've spent the last half hour trying to login and only got in once, then got 307 booted error while doing writs. WTF is going on? (win10 non-steam, Fios Gigabit, PC-NA)
Right now the steam servers are being overloaded because of their sale, even the website is throwing fits. I'm not sure if that affects ESO login or not though.
After I read a recent post in reddit, steam community forum and ESO forum about problems logging in to the game from Steam today on Saturday 24 June 2018 (only a difference of 5 minutes for each post
I played yesterday, everything was fine. There is a game purchased through the site, through steam and through the Sony store on ps4, I can not log into accounts. only on the forum the account is work