Well, like I wrote before, I recompiled all my KDE packages without the debug USE-flag. Sadly that didn't help anything! :( As a matter of fact it even managed to run my new HDD full - with 270 GB of .xsession-errors created in less than a day.
The solution was to emerge kdebugdialog. With this little tool you can set the debug-output of your KDE programs. After telling KMail not to cry about it's own incapability in .xsession-errors I am much happier and my HDD much emptier. :)