Response window (XML tab) has stopped showing any responses (blank all the time)

As the subject says, I no longer get XML responses, only the Response Raw tab shows the response (and yes, it is an XML SOAP envelope just to be clear).

I initially thought it was because the response was too large, so I increased UI Settings / Size of Raw Response Message to Show, but it still did not help; even for small XML responses, only the RAW response window shows data, the XML window is still blank.

Why is this suddenly happening, it was working as expected for more than a year !