Account failure: status unknown
I had to reinstall twice, because after some time (or maybe after force-closing the ring app) the account did not connect properly anymore.
The account was created for testing purposes, without registering a user name and with an empty password.
After the error occured there was an "!" next to the account in the menu. When checking the account details, disabling and re-enabling the account shortly displayed Ring-Account connecting, and then unknown.
Possibly related?: https://tuleap.ring.cx/plugins/tracker/?aid=893