Darrel talks about it in post 19 of this thread:

There is something seriously messed up with these chips. And it's all Microchip.

The SFR's from address F40h to F5Fh cannot be accessed with either the MOVFF opcode or indirectly through the FSR's.
They can only be accessed by setting BSR to bank15 and using "banked" opcodes like movf, movwf, bcf etc.

I realize that those SFR's are outside of the ACCESS bank, but that should not affect MOVFF or indirect addressing thru FSR's.

That range of SFR's contains many of the USB modules registers.
Other USB module registers such as UCON USTAT etc that reside in the access bank can be written/read with all 3 methods FSR, MOVFF and banked. So it's not related to the USB module itself.

The USB libraries rely heavily on MOVFF and FSR's.

I have found nothing in the errata or datasheet that mentions this behavior.
And in due time, we will see his work around!