I also noticed this yellow substance when I unhooked my high pressure hoses from the tee on the RV propane bottle. My Coleman Roadtrip grill quit working attached to the high pressure hose from the RV, but still works with the throwaway bottles.
Thinking that the yellow substance might have clogged up the valves on the end of the hose???
2018 Refection 311BHS
2011 Wildcat 32QBT (SOLD)
2006 GMC 2500, Airsafe 20K hitch, Air Lift Loadlifter 5000 air bags with Air Lift WirelessONE compressor, EFI Live
Jack of all trades, Master on none