This was a consequence of exactly following the procedure suggested by Microsoft:
- They suggest to first install only the CA without additional role services
- They suggest to edit the registry settings exoported/imported only regarding server names
As a consequence, I did not directly install the Web Enrollment. However, as I meanwhile found out, the registry contains a setting that states that Web Enrollment is already installed. Changing the setupstatus from 0x6003 to 0x6001 as suggested solved the problem.