Hi Fran,
It might happen to lots of different users on your Moodle site, but that still doesn't help to diagnose what is actually going wrong.
The error basically means: "there is something wrong in the response from the server"
To diagnose what that "something wrong" is, you would need to look at the content of the response and figure out why it is wrong. That can only be done by looking at the raw responses as contained in the "network" tab of your browser's developer tools.