Special characters
I like to create svg-fonts using the Custom Stroke Font extension and use the fonts with the Hershey Text extension. The enclosed svg-font generated using the Custom Stroke Font extension will cause Hershey Text to report: "Error parsing SVG font at C:\Users...\test_CutlingsGeometric.svg Warning: unable to load SVG stroke fonts."
So far I have solved this by replacing all special characters with hex-code (see blog post). As I understand your blogpost-comment this error should not occur for special characters like f.x. Å (Aring). I have also tested with a copy of the enclosed file where I have these glyphs only: 0-9, a-z, A-Z and Å. This causes an error, too. Removing the Å or replacing it with hex og html code did not cause an error.
Using Inkscape 1.0.1 (Hershey Text 3.0.4) on Windows 10.