Ditto john_s report that it's the acid in acid fixers that causes the mucking problem with KRST.
I use alkaline fixer (TF4)..single bath. I rinse it shortly in my print washer after fixing, then move it directly into the KRST for toning. The intermediate wash is enough to rid the print of any residual developer (when the fixer is a tad used). Developer in or on the print (even at very low concentrations) will seriously muck up KRST.
Using alkaline fix greatly simplifies things, IMO.