I would suggest using 'from Database' as source if you're not setting any XPath constraints. That will probably help you fill up the grid again.
Apart from that: The requests being done from the client to the server (and the responses) are in JSON, hence the response message.
This problem has been opened as Issue with support. An other user experiences same problem, Possibly this has to do with handling of virtual attributes in 7.6.0. In my case it occurs with multiple entities..