125' + the length of the charging cord. I'm not electrician, but there is definitely a limit on the length you can run. Maybe one of the more expertise posters can chime in.
I know when I first installed my 14-50 outlet, I used 14' 6-3. Then I later added a 35' RV extension cord. The first couple of times I used it, for some reason, it gave the same error you got. I had to move the amps to 32. But then one day, I decided to see if it would work on 40 amps again, and lo and behold, it did. Never had the issue again. I'm not exactly sure what changed.
125' should still be ok with AWG6 cable. Esp. at only 32A. Look at what the car reads for voltage during the charge. If it's really close to 240V, you're good. If it's under 235V, I'd say you're losing an unacceptable amount of energy (and money) to resistance in the wire.