500 error when editing/adding content after upgrading 2sxc to 19.03 LTS

[webservices94]

We upgraded 2sxc from version 17.9.0 LTS to 19.03 LTS on a DNN 9.11.02 install, but now we encounter the following error popup window when trying to add/edit content types or add/edit fields in the 2sxc admin:

Had an error talking to the server (status 500). Message: An error has occurred.  
Detail: The 'ObjectContent1 ' type failed to serialize the response body for content type 'application/json; charset=utf-8'.

We have tried restarting the application/reapplying permissions, and we also tried the same upgrade again in a staging environment and had the same result.

Could this error be due to a missing intermediate upgrade (e.g., 18.x)? If not, doss anyone know how this error can be resolved?

[iJungleboy]

This is almost certainly not related to the direct update - the update path you describe is very common.

Every request is logged to the Insights. Could you research that to see if the insights reports more details as to what it is doing? Because apparently it's trying to give the browser some objects which seem to cause trouble serializing - or the browser is sending something which can't be deserialized...

[iJungleboy]

I believe it's a threading issue. Will create an issue...