I am also stuck on this.
If you "logout" like Support's standard dismissive response to every bug, you will be stuck in an infinite "uknown error" message.

I can't log into main character anymore.
I can log into mule characters just fine.

Got it, and you know the best part? I have an alt in Daggerfall for whom the quest worked just fine a couple of weeks ago. So now I'm really, really curious if they managed to break it with one of the patches(I thought those were supposed to FIX things but I guess I could be wrong, maybe that's the old-fashioned way no one ever uses any more) or there's some gigantic yet unnoticeable difference between the vet and non-vet quest.
Tried abandoning, 0 result>.<

Bugged quest, can also confirm. If you even enter that cave it continually lags too, something seriously wrong there.

I have this same problem, I went into the cave that the quest is in and logged out and now I just get error messages on that character, how do I fix this?

Lagged quite incredibly and none of the souls of the Three could be activated. I've done it successfully before with other characters. Pfft.