RootCause: Invalid JSON - answer seems not to be from translate5 - x-translate5-version header is missing

A lot of error messages were reported by customers via RootCause, saying

Invalid JSON - answer seems not to be from translate5 - x-translate5-version header is missing.

Example https://app.therootcause.io/#marc-mittag/translate5/errors/9e607935cfa41d148e34eb84ca7684a56792c016

Those errors pop on different environments (OS and Browser) and during various operations initiated by the end users, so the impression is that it's something server-side related, but not related to expired user session as this is handled separately