This is an indication that the site has been subject to a brute-force login attack, specifically on the Connect Manager (downloader) login.

var/brute-force.ini should look something like this:

brute-force-bad-attempts-count = 0
brute-force-diff-time-to-attempt = 180
brute-force-attempts-count = 3

Any value for brute-force-bad-attempts-count > brute-force-attempts-count will result in the above error.

Solution: edit the file to set the count back to zero, and remember to rename the downloader directory to something less obvious when not in use.