I do not have logs from HA anymore, but using your API without a higher timeout (minimum 15s), the same error as reported in this issue appears. After further investigation, I found out that XMO_AUTHENTICATION_ERR is returned by the router.
It seems like those Telekom routers need some more time…
The reboot service just did not work for me, but no errors in the log. With await in front of client.reboot() the router restarts as it should.
Unfortunately I don’t know how to create a pull request properly, I mainly program for myself and have a very rudimentary knowledge of GitHub, sorry