A screensaver no longer works in XP

I have made a few screensavers with Axialis' Professional Screensaver program.

A couple of them have been put into the system32 folder, where all the others reside.

Until recently, both of them worked. I now find that one of them doesn't. It functions correctly in the Preview facility in Display Properties, but, despite pressing Apply, and still appearing in the screensaver list, it doesn't work.

On looking again at Display Properties, I find that the screensaver setting is at (None).

What's gone wrong?