I've checked my log, with "https://" it won't be able to contact the remote through the watchtower_url:
Without "https://", there's the same error; but, the watchtower recognized the client's channels either way.

I think "remote watchtower url" isn't necessary for localhost since the client and watchtower are both running in the same machine.
As long as "use_watchtower": true, is also in the config, it should work. (I'll edit the client setting in previous reply)

But to be sure: you can open an issue in Electrum's repository here: https://github.com/spesmilo/electrum/issues
And paste the errors when localhost is used as "watchtower_url" in the client.