What version of the STP protocol are you using in your Ubiquiti switch? If it is set to RSTP change it to classic spanning tree (802.1D). I have experienced the exact scenario you are describing with nearly identical equipment.

I don't know your specific network topology but I would assume you would want your switch to be the root bridge as it is likely directly connected to your router. The root bridge should lay closest to the destination (router in your case) in order to provide the least amount of hops for data to travel. You can force the switch to be root bridge.