First GCM registration after installation fails
Since CalyxOS 2.1.0, the first GCM registration of an app after a complete wipe fails, second try succeeds. I tried with Signal, which does not detect GCM on the first try, tells me that Google Play Services are missing. Trying again after reinstalling Signal fixed the issue.
I also tried with the german DB Navigator app, same behaviour.
After the first fail, every attempt to register an additional app seems to work. It's really only the first registration after a wipe that fails.