I ran into this issue and was able to "fix" it by turning sync off, then back on for ALL layers included in the web map, make sure any tile packages included in the map or basemaps have Offline Mode enabled:
"Offline Mode
Allow this layer to be downloaded and used in an offline map."
Unregistered all the Replicas created for ALL feature services in the map,
and, for some reason, I had to select the "use in Collector" and the "use in Field Maps" for it to start working again.
Not sure what (if any) of these things resolved the issue, but, it is working again now.
R_