Help please: Value cannot be null. Parameter String.

Hi,

I'm having issues with Sharepoint 2013. I'm getting errors such as below, using Fiddler I've identified that calls throught things such as the "Upload a document" button go into "/my/_vti_bin/client.svc/ProcessQuery":

{"SchemaVersion":"15.0.0.0","LibraryVersion":"15.0.4420.1017","ErrorInfo":{"ErrorMessage":"Value cannot be null.\r\nParameter name: String","ErrorValue":null,"TraceCorrelationId":"f23cf99b-e744-1048-59ff-a083bb351f02","ErrorCode":-2147467261,"ErrorTypeName":"System.ArgumentNullException"},"TraceCorrelationId":"f23cf99b-e744-1048-59ff-a083bb351f02"}

What is 'null' here and any ideas how this can be fixed?

Could this be related to the proxy we just set up? If so, what has gone wrong?

Thanks for any assistance,

Matt