Unsupported format error

[vaguely]

I have been getting the following error for the last 48 hours or so one of my sites for a sitemap.xml.gz file:

Unsupported file format
Your Sitemap does not appear to be in a supported format. Please ensure it meets our Sitemap guidelines and resubmit.

I am saving the file exactly as I always have and have never had this problem before. Have regenerated it several times to no avail. No configuration changes.

Plain xml and urllist.txt are accepted without issue.

Ideas?

[XML-Sitemaps Support]

Hello,

what is the sitemap URL?

[vaguely]

Never mind - seems to have cleared with a new file.

The previous files, when unzipped, were found to contain just one line of nonsense. Only happened over the last 48 hours or so and now cleared up.

Will let you know if it recurs.