fckeditor file browser not sending proper xml response

A site I help maintain suddenly stopped letting the client link to files using fckeditor, alerting that "The server didn't send back a proper XML response" when the file browser was opened.

This is one of the rare occasions Internet Explorer returns a more helpful error message than Firefox - it included the information that the XML request error (actually the http response code) was OK (200).

After initially checking such thing as server settings and response-type/encoding headers I set to perusing the actual returned xml in Firebug and spotted a suspicious looking square where there should have been a regular character. Turns out the client had uploaded a file containing a character which must not exist in utf-8 - specifically an em-dash ( &mdashl )). The xml lists filenames as attributes of "file" nodes, so the browser was getting to this character, failing to recognise it, and declaring the XML response invalid.

Changing the filename resolved the problem. I didn't encounter this possibility described elsewhere while I was googling, so here it is.