I googled and found many reports of the “unable to deposit items into guild bank” problem. In each of those threads, a Technical Support forum rep popped in, said they were looking at the problem, then popped back in later to say it was fixed, which is then confirmed by others in the thread.
I think Tech Support might be the way to go on this, barring me being oblivious of some more obvious issue. 
Some info: Guild bank - no access!
Edit: There’s also quite a bit of info in Bug Report, though the threads boil down to the same resolution. I’m sure it would also be useful if you put in a bug report of your own.