Can't add second google account to google calendar app
So the solution for me was: 

First activating Device Information in my Google Activity Settings.

Second I deleted the account from the phone and added it again. There was the toggle for syncing the calender.

Third I needed to allow google calendar to have access to the calendar storage, which was prohibited by default from the Privacy Guard in CM. This solved the sync error problem.

Now all my calendars and events show up on my phone.