I tried several things last night. The fix most probably wasn’t the change of my email address. Let me try to sum up the things I did. 
In the account that I could not login to central with, only had access to discourse and the Keycloak account management (I checked that in “Applications” in Account Management).
I created a second account and could log in to the the central application. In Keycloak, I could see I had access.
I could log in to both accounts so my plan was to change the email addresses of both accounts so I could use the preferred address at the working account.
I noticed the e-mail verification was not triggered immediately, but at log in (via Google) to the central application.
After the verification of the email address the registration flow to grant access to central seemed to work or the grant happened in the same process (not sure what the flows in Keycloak look like or if it is even managed with flows ;-)).