Earlier versions of the Engine DJ OS included a crossfader enable/disable setting in the Control Center. However, because the PRIME 2 also features a physical crossfader assign switch on the front, the internal setting was removed in Engine DJ OS v2.1.0.
If the crossfader is disabled in the Control Center prior to performing the update, the crossfader will be stuck in a disabled state.
To correct this, a previous iteration of the Engine DJ OS will need to be installed on the device and the state of the crossfader will need to be changed before upgrading back to the latest Engine OS.
The crossfader should now be functional again. To disable the crossfader in Engine DJ OS v2.1.0 or higher, set both crossfader assign switches on the front to "Thru".