My minion vault is full.
I tried opening a gold chest with a key, and got a pop-up: minion vault is full, go salvage something.
Then it dropped me back to the “deal with your chest” screen.
I was lucky. I had a spare chest slot to use while I figure this out. I usually don’t.
I think it should have dropped me to the minion vault screen, or the check should happen when it unboxes the minion from the chest and then gone to the minion screen.
I would have been very sad if all of my chest slots were full and I lost a whole chest just because my minion vault was full.