Thanks for the information!
That made it easy to solve the issue.
The problem were whitespace character in the author and/or license fields.
This is fixed now.
The upload filename doesn't matter, it was already escaped.
The download filename is automatically converted to match the LibreCAD reqirements. Whitespace characters are removed and the filename all lower case. Otherwise LibreCAD can have issues to find the font.
I got it working consistently - thanks for the tips.
Single stroke TT fonts are hard to find. I had read about the issues with outline filled fonts, and after converting a few am seeing it first hand. I have an AdobeCC account, but didn't see any way to find the stroke fonts - and I doubt their license would permit conversion...
Is there any way to convert an SHX font file? I have my old ACAD fonts.