Huh. Ran into this behavior yesterday, coincidentally. I also think there's another bug with xscreensaver where it will capture and "hang on" to your keyboard after you've unlocked the machine. Maybe it's time to switch...
You sure you are running the latest version?
That is the origins of this message. That Debian (and perhaps other distros) would fail to push xscreensavers in a timely manner, resulting in JWZ getting emails about issues he had long since fixed.
This seems to stem from a policy that only security issues (resulting in a CVE being published) will be processed, while usability issues are left in place until the next major stable release is rolled out.
I can see two reason for this.
A: that the thinking around stable is heavily server oriented. Thus anything but CVEs being excluded result in reduced risk of production breakages. This even though Debian is a generic distro that can be molded into desktop or server usage depending on what gets installed.
B: that the rigidity of traditional package management do not allow a piecemeal updating process. This because updating a package removes the old package in the process. Thus if you want to bump up the xscreensaver version, and it hard depends on a newer lib somewhere in the dependency chain, you end up with an unresolvable conflict unless you also update everything else that depends on the same lib.