Sometimes Workrave immediately assigns me a rest break after Windows XP Professional starts up.

The environment in which the problem occurs is a small business network with a personal login account to a server, meaning I can also login to to other pc's with the same network, while Workrave is installed at a computer specified harddisk. Workrave is in the startup items. This problem has never occurred yet at my home computer, which boots Linux Ubuntu 8.04.

The problem is not easily reproducible, as the mentioned error doesn't occur every time.

Workaround: skip rest break. Functionality loss thus depends on the possibility to do so.

Does this happen immediately after a reboot? 

If you reboot your computer, and the total reboot time is less than your restbreak time, Workrave will not reset its timers after a reboot. So if you are close to a restbreak, and reboot, Workrave will still be close to a restbreak.

Same thing will happen if you logout and login on a different computer (assuming your document and setting is stored in a server)

Could this explain the behaviour?

Yes, it's immediately after a reboot or a login. I think your explanation is probably correct. Still, it's a bug, as it's clearly unwanted behaviour. It's logical that you can't get around Workrave's timers just by restarting your computer. I understand that. However, it's quite annoying when you start or login to your office computer Monday morning first time after the weekend and Workrave tells you a rest break is necessary. That's just plain silly. For me personally, it's only annoying. I trust my self discipline, so my Workrave settings allow me to skip rest breaks. This issue hampers the functionality of more restrictive settings, though.