This is a bug that was recently fixed. If you use Rosetta 3.9 instead of Rosetta 3.8, the bugfix should be included.
If you want to keep using Rosetta 3.8, you’ll have to edit the source and recompile. Go into Rosetta/main/source/src/protocols/md/CartesianMD.cc around line 954 or so, and change the line
attlist + XMLSchemaAttribute( "scorefxns", xs_string,
to be
attlist + XMLSchemaAttribute( "scorefxn", xs_string,
(Remove the ‘s’). Then recompile with the same scons command you used before.