Some 3.5mm → 1/4" plug adapters are poorly manufactured and may be hard to insert into some 1/4" jacks. I have come across couple of those during my career. It’s all about manufacturing tolerances: jack is little bit too small and plug is little bit too big and you have a conflict.
Go to your local Hi-Fi/Electronics dealer and buy a new adapter. You may even bring your UR and headphones with you to the shop in order to make sure they connect into each other.