If they are fixer stains (dark grey-brown-black?) the stains don't do anything and are safe for further washing. If you are obsessed with getting the washer clean, sulphuric acid works, but read the MDS and take appropriate precautions. I also don't know the washer - you can't safely expose metal to sulphuric acid.
Edwal used to make a product for this purpose but it seems to be discontinued (B+H no longer stock it).