Can you look into log files to see if there is any stacktrace in there?
One thing you can try is to add this setting to appSettings
<add key="aspnet:MaxJsonDeserializerMembers" value="2500" />
I ran into this issue as well and was able to add that key and have it resolved. Kind of an odd fix but I will take it.
We are facing issue while adding a gadget 'External review component' in CMS, it throws error as 'Could not add component' pop up.
EPiServer.Global : Unhandled exception in ASP.NET
System.InvalidOperationException: The JSON request was too large to be deserialized.
Kindly let us know how to overcome this issue?