It seems that the bank bug is alive and well. I was switching characters at about 11:45 this evening. When I logged in on my Breton I received duplicates of Imgakin, the Explorer's Pack, Exclusive Digital Content, and Exclusive Digital Content--Nibenay Mudcrab in the mail. When I checked my bank, it had shrunk from 100 slots to the starter allotment, and the only item in it was my original Imgakin.

Really a fun killer after an evening of farming herbs and runes. And just getting my stuff back won't really help unless they can assure me that they've solved the underlying problem. Pretty bummed ...

That sounds absolutely terrifying. My bank is packed full of rare items.

Is there any known way to help prevent it or anything those who have been affected have in common?

@Imryll

Happened to me too tonight, BUT ...

i immediately logged out and then logged in with an alt char and the bank was back. I took out the important stuff and relogged with my main char and the bank was back for him too!

However, i did not have any extra bank slots so i don't know if i would have been truncated down or not.

Also noteworthy, i now have a duplicate of every explorer pack item ...

I logged back in and my bank had 55 items in it. Many additional items had been added to the inventory of the alt I logged in on, so I seem not to have permanently lost all that many items. I am still missing 40 bank slots, though, and I see posts in the Customer Service forum by folks reporting that this has happened to them two and three times. At least I didn't have any gold in the bank when this happened!

Thanks for encouraging me to log back in, SirAndy. I do feel a bit better, now that I know I've not lost all my crafting supplies. Time to create another mule or two, I guess.

Exactly what i did. 3 main chars and 2 mules.

For now, i'm going to leave the bank vanilla and won't upgrade it. I hope this will minimize any impact from this bug ...

Of course, as part of the reset any mules I now create will have to run through Cold Harbor. Apparently the game doesn't consider having higher level characters to be evidence you've experienced Wailing Prison. Normally, I do it anyway, but I've yet to run the Wailing Prison without beginning to feel a level of engagement that I don't want to feel with temp mules. Mule 1 is already level 5 and has taken on experimenter with bows duties in addition to her storage responsibilities.

Funny, same thing happened to me. I create a female Khajiit mule and by the time i was through the prison, i was like:
I really like her, i want to play this character