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.
You can download the LFF from my test directly:
https://librecad.org/ttf2lff/uploads/architectsdaughter-regular.lff
investing less than half an hour into Search function can save hours or days of waiting for a solution