Error on document upload

Hi,

We are receiving the following error when uploading documents to SharePoint 2013 sites. This error seems to be occurring on drag/drop only. Uploading via the interface and windows explorer does not seem to be an issue. Restarted both web front end servers but no luck. Any ideas?

Error with request to the server: Unexpected response from server. The status code of response is '500'. The status text of response is 'System.ServiceModel.ServiceActivationException'.,  StackTrace:

thanks,

Sherazad