Carsten,

it seems that the newest version of Orgmode (6.15a) has a bug: When I try 
to move a tree/subtree to the archive (C-c C-x C-s) I get this message

Wrong type argument: number-or-marker-p, nil

and the tree/subtree isn't moved.

The entry in the menu (Org --> Archive) doesn't work either.

Schöne Grüße,

Uwe