This seem to have started about a week ago, after the last maintenace window. I will end up stuck in a load screen. Sometimes I see the, "unusually long" message, but sometimes I don't see anything happening other than the spinner. Maybe 3 times I have gotten a screen that was entirely black, except for the spinner.
I would say it happens most often when I logout a character. Next most frequent, it can happen after clicking 'play' but before getting to the character picker. It has also happened after picking the character and trying to load into the game. I have not had any problems whiloe zoning into dungeons or using wayshrines.
If I wait long enough, I will usually get kicked to the main title screen, with a message about inactivity. Right now I see error 308, but I couldn't say for certain that is always the error. There are no problems with my connectivity outside of ESO. I have run speedtest from IE and the Xbox troubleshooting tests also, and everything says my internet connection is very good. (Typically I see 100 Mbps down / 11 up.). (For example, when I get frustrated from this, I can go watch Netflix without any problems.)
Right now, things started with a hang while logging out. Since then I am unable to get from the 'play' screen (announcements) to the character picker. Choosing to 'Quit' the game has not had any effect. Read an older, similar bug where someone sugeseted loging into the other server, e.g. I play NA, so try logging into EU -- this had no effect and it took a long time to even change the server. I finally did a hard reset of Xbox and I am able to login a character again.
I feel like began with the last Xbox maintenace (about a week ago), definitely not more than 2 weeks.
Since the Aniversary event began, I usually login every account at least once a day, which is unusual for me. (Normailly, I would use only 1-3 characters on any given day.)