Due to other reasons, I tried to not use migration assistant to import a user of the old system.
Do return to normal-operation mode, I ran migration assistant. While Apple.Mail's import feature failed, migration assistant managed to setup the old user with a fully identical mail configuration.
While I still would be happt to learn how to fix the above mentioned Apple.Mail problem, I'm going to use the imported user.
PS: Since I created on the new system a case-sensitive HFS+ system volume, certain apps failed to install. Using iPartition, I was able to convert vom case-sensitive to non-case-senstive in 15 minutes.
May this conversion be somehow harmful? While the system works right now, may it render problems sometime later?
iPartition was even able to create a bootable recovery system on a thumb drive with iPartition included in the list of applications. Nice!
PS: One reason, why I tried to not use migration assistant: It created a new ACL Group for an application - which isn't any longer present on the new system.