Hi Phil,
The only time I’ve seen this error occur before, the site owner had his webserver and redirections set up wrongly, and the webserver was stripping out parameters that were being passed to the backend. e.g. https://example.com/blah/blah?x=y&a=b&c=d was being passed to the back end as https://example.com/blah/blah?x=y. Does that make any sense, and does that ring any bells as possible?
If not, then is it possible to send me your WP + FTP login details? (To [email protected]) ?
It’s the end of the day here, so please excuse a lack of replies until tomorrow!
Best wishes,
David