I appreciate the help, but I don't think I'm going to spend any more time with this right now. I have to work on the document itself, not the LaTeX-details.
Actually, the number didn't disappear, it just appeared on the top left for some mysterious reason. When I tried to place it there intentionally with \fancyhead[LE,LO]{\thepage} the page number really vanished. :)
If you know some nice way to fix this, that would be great, but if not I'll just ...
I compiled your minimum file, and the page numbers appeared just like they are supposed to.
After some testing I discovered that the problem wasn't caused by savetrees, but by combining savetrees and fancyhdr. In the following example I get the page number 1 on the first ...
Tried to search for this, but for some mysterious reason there is a character limit in the search bar.
Anyway: In my document, I would like to have a very specific page break rule at certain points: If it's less than half a page left, then start a new page .