Apparently our subscription isn't associated with my user/email address, so I can't submit a ticket myself. I'll have the guy who bought the license submit the ticket.
I saw a 4 year old post here for the same error that indicated a solution involving adding a web.config appSettings key to overcome a limitation in the .Net framework on the JSON request: <add key="aspnet:MaxJsonDeserializerMembers" value="150000" />.
But this didn't work for me when I tried it, and I can't help thinking maybe the 4 year old post was for an earlier version of NOP that wasn't in .Net core, so the fix might no longer be applicable.
We're on NOP 4.0. Does anyone know what else might cause and/or fix this error?