S5 - Suggestion
I have entered something in the properties score in the source, when uploading the field was automatically filled with the HTML command line. I suspect that this is the cause that when uploading the Musecore 3 program crashes. In addition, the Muscore page must be opened in the browser with login for the upload to work.
Score needed. And no, nothing entered into File > Score properties > source should cause a crash
For the uploading via command line you need to first once use File > Save online once, for the credentials to get stored