Is in kmail a possibility/option to write debug-logs ? To get a chance to send some info ?
Sure, there is. There is the filter log (see the Tools menu), then you can enabled the different log areas for KDE applications with "kdebugdialog". Here you should enable the one for the akonadi filter agent (and disable most of the others to not get a too big log) and restart akonadi from a console with "akonadictl restart". You will see the debug logs then.