What fixer did you use for how long? How fresh was it?(posted 8370 days ago)Other two mentioned possible problems but I think those problems take time to become apparent or entire image deteriorate at a similar rate rather than forming spots.
My speculation is that the stop bath and/or fixer weren't fresh and developing agents carried over to such stages caused weak stain. The stain is often clearly visible, but it could be weak. Easy solution may be to use fresh chemicals, and make sure to stop each print for several seconds.
Buffered stop bath (those typically contain BOTH acetic acid and sodium acetate) is usually safe to mix a few times stronger than recommended and they work fast and last long. This frees me from trouble of checking stop bath's pH often.
If the fixer is old but hasn't reached its capacity, it may be helpful to add up to 20g/liter of sodium sulfite, but you might not want to bother unless you have sodium sulfite handy.
Finally, make sure you wash prints well, using clean rust-free water and apparatus.