Yesterday I installed Ubuntu on my pc, next to Windows, to create a dual boot system. I have been using Rainlendar (registered) on windows for quite a while. I also installed Rainlendar on Ubuntu and used the same license file I used in Windows. No problems so far.
When I booted up in windows again, Rainlendar mentioned the text [Unregistered] on the calendar, each event/task. So I used the same license file on the Windows installation, again.
Now on both Windows and Ubuntu Rainlendar works without a problem, fully registered.