After starting KMail, once can see the main window, but then KMail freezes for a few seconds. I _guess_ this is when it tries to load all folders, so the bug only happens when having many folders. Once can also observe that before the freeze, some folder names are empty, and they are correct after the freeze. The folder list gets resorted as well.
Probably related: bug 223471
*** This bug has been marked as a duplicate of bug 223471 ***