Hello Jaap,
I've been working on this bug that the system breaks are lost upon reopening.
The file
Als sterren aan de hemel staan.not that you sent us definitely shows this problem.
I've tested the problem opening a fresh MIDI file, which doesn't have system break data already in it, of course, because that data is only kept in .not files. I'm not seeing the problem in that case. Well, I'm not seeing it now. When I was investigating the bug a while ago, I thought I saw the problem also with fresh MIDI files.
Can you reproduce the loss system break when you start with a fresh MIDI file, add some system breaks, save the .not file, and reopen it?
Cheers
-- Mark