I have a 1997 Volvo 850 T5 experiencing auto transmission problems. Lately, after draining the ATF to facilitate rep;ecement of the drive shaft, I had the transmission filled with Volvo original ATF Dextron III.
Now the reverse gear won't work at all. Other gears are working fine though. I have double checked the ATF level with the engine running and it shows sufficient fluid level at the current temperature. There is no sign of oil leak.
It appears that you did not touch the transmission at set to refill it. I think the problem may be just a stuck valve and would mean taking down the valve block and cleaning it out. It may be just dirt that has jammed a valve. I suggest hat you get someone familiar with A/Ts to have a look at it.