Does your 855 have a PTO brake? If it does, make sure it's working correctly. It might be necessary to physically disassemble the brake to check it out.
/ 855 John Deere PTO engage engine lugs and stalls
I have the same problem with my 855. The shaft turns easily by hand when selector is set to that shaft. Any idea what it may be? I believe seat switch works because nothing will happen if button is left up.