Found an ugly solution:
1. Deleted the mailfolder ~/.kde/share/apps/kmail/mail/
2. Started kmail and let create the "Local Folders", which now point to ~/.kde/share/apps/kmail/mail.directory/
and created another folder
~/.kde/share/apps/kmail/mail/
3. Then rsync all mails from computer A into folder
~/.kde/share/apps/kmail/mail.directory/
of computer B.
4. Restarted kmail and suddenlky all mails appeared again!
This is working now, but left some questions:
1. Why can the new folder not be named
~/.kde/share/apps/kmail/mail/ and is called */mail.directory
2. Why does kmail on two computers not behave identically, although all configuration files were identically on computer A and B and the complete akonadi database was deleted on computer B?
3. Why did "Local Folders" NOT read my mails, although I pointed them to the folder, where my mails resided?
4. And last but not least: Why did it read from the new created folder "*/mail.directory/", although this was identically to the former folder?
Maybe someone can answer this question, however, if not, it will be ok for me. Since akonadi was introduced in kmail, IMO it is a pita. If it works, you are lucky, but if anything is going wrong, you are getting in big trouble.
But maybe I am just to stupid for it....
Anyway, problem (ugly) solved.
Best
Hans