User Picture cannot be changed

The problem: The user picture cannot be changed. After uploading the initial picture only this is the one which will be displayed. Even if I delete it, clear cache and then upload another picture only the old picture will be displayed.
I also checked that after the deletion of the old picture the databasetable got it right(=showing no entry under picture).
When I upload a new picture the database correctly gives the path to the picture. My understanding is that drupal gives the picture an number which corresponds with the UID. So my User Nr.13 has always picture 13 in his profile. While in the folder sites/default/files/pictures the new picture 13 can be found the old one is displayed eyen though it is not stored in /pictures anymore.

So at the moment my options are only the picture first choosen or no picture.

I am using Advanced Profile Kit 6x-1.0 Alpha1, Author Pane 6.1 beta 2, Panels 6x-2.0 alpha 3.
My system is a XAMP localinstallation.

I could hardly be an permissionsissue because I also tried it as superuser.

Has anyone a clue why this is happening and how I can correct it? This really drives me crazy.