Hi @fxholl, this looks like a problem with the tunnel URL provided by n8n. n8n cloud (and self-hosted instances on a webserver) would not use this tunnel which is why the authentication would have worked fine there.
From quickly testing the tunnel from my side it seems to be up and running, so I suspect the problem could be with your specific tunnel URL. Perhaps you could stop n8n desktop and try resetting your tunnel URL as described here? On the next launch, n8n should request a fresh tunnel URL which hopefully works as expected.