Have you found a resolution to this issue? I am experiencing the same issue with a web map I cannot view in Field Maps with the same error code "Unable to Open JSON parser invalid token" and I can also not view the map in the new version of collector and received the error message "Unable to Open Map invalid call with current token type"
However, I am able to view the map in the old version of collector.
I do have an idea, but my solution may not be applicable to all. My maps in Field Maps also gave the same error code, "Unable to Open JSON parser invalid token", and I believe it has to do with a custom Arcade expression either within pop-ups, style, or attribute table. I removed the Arcade expressions from each while within the web map, but was still unable to open the map in Field Maps. I had to remove the offending feature from the web map in AGOL, make all changes to the feature layer default display and table, and then reload the feature into the map. I wager you could pinpoint exactly which Arcade expression that Field Maps does not support, but my solution was acceptable for issue at the time.
In my situation, the web map had a problematic feature layer. A new map was created and the layers were re-added to the new web map. The new web map had no issues being viewed in field maps.
We just struck the same issue in ESRI Enterprise 10.9.1. Again it is related to the popups.
The layer was a SQL view registered with the ESRI SDE Database and published via ArcGIS server into the Enterprise Portal.
Using Map Viewer Classic, and NO popup enabled, the map will load correctly in field maps.
Using Map Viewer Classic, and Enabling the popup, the map will NOT load in field maps.
However, if we used the New Map Viewer to enable popups and save the result into a new map, Field Maps could successfully load the map and show the popups.
@PaulBarr This solution seems to have worked for me. However, any layers that were an SQL view did not load (a warning of 'Layer failed to load' show up in the layer list). Any idea why the SQL view layers won't load in Field Maps?