OK, now that it is established that the mower lift works with the FEL on but doesn't work with it off, there is only one malfunction. That is, the hose connections where the FEL hydraulics attach (as shown in TripleR's photo) is somehow not connecting.
After all, the FEL is just one giant loop in the hydraulic system. To take that loop out and "short circuit" by making a smaller loop should make no difference in operation of the rest of the hydraulics.
The connectors must work because they work when the FEL is attached. Soooooo...check the short circuit loop once again to make sure it is fully connected. There can be no other reason.