In solara.server.flask line 202, in nbext(), the Flask url that is passed into send_from_directory is improperly formatted using os.path.sep, which is different on Linux/Windows. According to werkzeug, this function apparently actually accepts a URL path, not an OS path, which should always use / regardless of the system. (Currently on windows it uses \, which causes the path to be blocked by werkzeug under the hood).

I propose changing the following line:

return send_from_directory(directory, dir + os.path.sep + filename)

to either of the following:

return send_from_directory(directory / dir, filename)

return send_from_directory(directory, dir + "/" + filename)

Both fixes fix the issue when running on Windows.

See pallets/werkzeug#2927 for my issue on werkzeug's repo and the owners' response.