Look, this is easy.  That sort of stain is always caused by carryover of active developer either by not using a stop bath or having dirty finger or tongs.  There are a few ways this can happen.  Going straight from developer to fix will cause an overall fog, a situation which is exacerbated when the white light is turned on before fixing is complete.  Stains at the edges of the print almost always point to developer contaminated tongs or fingers handling a print that's not completely fixed out.
Bottom line, you really need to use a good stop bath, make sure that your fixer is good, and that your hands or tongs are not the cause of cross contamination.  There's no getting around it, and it's not a bad batch of paper.  If the paper is fogged, the stain woudl be black or grey - not yellow or brown.  You can use a water stop, but that takes lots of time and lots of water to ensure that all the active developer is removed from the print.  It's not practical, nor is it as effective as using a proper stop bath.  The yellow dye in an indicating stop bath will NOT stain your prints.