oblivious KMail (1.9.1)

Hi all, since some time my KMail 1.9.1 (i'm using it embedded in Kontact) doesnt remember some settings. So i have one Folder, which is filled through a filter, and i want to have it to delete messages after one day. If i make a right click on this folder and then click on "Expire...", i can set it to "1 Day" in both boxes, activate them and also activate the box for deleting these mails. Sometimes this works for a couple of days, sometimes only until next Login. Then it comes with the default settings (all settings are greyed out and if activated, they show "30 days"). What can i do? Thanks, Jens