Field Maps Layer Fails to Load - Error Code 21: Invalid JSON

[NickAddison]

Uploaded a map to Field Maps and all the other layers load except one; the following error is given:

Domain: com.esri.arcgis.runtime.error

Code: 21

Description: Invalid JSON

The only thing I can think of that is noticeably different about this layer is that it has a litany of attribute rules tied to it and several field types that not accepted by Field Maps (Date Only).

Any thoughts on how to address Error Code 21?

Thanks for any help,

Nick

[JoabelBarbieri]

Same here

[TaylorBlose7]

I'm encountering the same issue with a feature layer and views off that particular layer. 

[Sietse_de_Haan]

I had the same problem today, Error 21. I removed the DateOnly field and created a new field with the formate Date. This fixed my problem.

I also had a warning when creating my form that there was a field that wasnt supported in FieldMaps (DateOnly field). 

Hope this helps

[KenBragg]

Good answer which helped me, thanks a lot.  Also seem that TimeOnly is not supported