I found the issue!
I was using
HfFileSystem('my_token_here')
instead of
HfFileSystem(token='my_token_here')
The lack of argument token made me unauthorised to access my repo, which gave me this 401 and weird url ‘preupload/main’.
So: explicitely pass the token argument, or check that you are logged in.
This url seems to be used in the background to pre-stage commits.