Hello Charles, thanks for your bug report!
You should install the solfege-doc package to get the html files that are referenced by the help link. I agree that if the documentation package is not installed then the 404 html error is not informative. A better behavior would be to display a nice error pop-up to indicate what to do. In addition, the solfege package does not Recommends the solfege-doc package, which violates the debian policy [1]. I'll try to upload a new package to fix these errors. In the meantime you should install the solfege-doc package to resume your work. Have a nice day, François [1] https://www.debian.org/doc/debian-policy/ch-docs.html#additional-documentation
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