Bank slot bug
If i have 99/100 items in my bank and then try to deposit a stacked item i.e. fire essences, the bank rejects the deposit saying that there isnt enough room as if i am depositing more then 1 unique item. I have found that if i remove 1 item from the bank to make it 98/100 i can then deposit my stacked essences. This must be a bug.
You probably have a container or bag with something in it, putting you over the 100 item limit. I forget how, but there's a way to pack more stuff into a chest than it can hold by using bags and such.