Is the tensioner pulley sitting on an axis parallel to the crank?

Either the tensioner arm is loose or the tensioner pulley is loose. Should be easy to send a 6mm hex bit into the tensioner pulley to check that. I can send a long 12mm hex bit into the tensioner arm without removing the fan.

There's a thick washer/spacer between the tensioner arm and the front cover. I don't know if the tensioner arm will pivot if that washer/spacer is missing. Do you know for sure that yours is in place?

There is a special tool that holds the belt to the water pump pulley so you can remove the fan. Alternatively there is a hole in the water pump pulley bolt pattern. If you're lucky, your water pump will have a big rib against which you can fit a 3mm hex key to lock the pulley while loosening/tightening the fan.

Release the shroud from the radiator and drape it over the fan for better access to the fan/clutch attachment bolt. Then remove fan and shroud together.

Sixto
93 300SD 3.0