I had the same issue at my organization. Its a shame it doesn't give more detail than just - "Unable to Open. Invalid Argument".
We fixed it by deleting the field maps folder in iOS (make sure there are no other pending edits on any offline enabled maps) and re-installing the application.
the first time I saw this I was still developing the web map, so I just rebuilt it.
this AM our field staff reported seeing the error; in the office, the map was fine.
I had field staff delete/reinstall Field Maps and they report it's back in order
thanks @gis_KIWI4 for the tip!