Switchtheme only worked once and then not again.

First I tried switching zen/zen fixed. I switched from zen to zen-fixed successfully, but there the block with the switch wasn't displayed again. When I went to the theme menu, it said zen was selected, but neither saving nor trying to switch back and forth via zen-fixed worked in any way. Only when I disabled zen-fixed, zen was shown again.

As I thought it may result from a zen problem, I tried zen/box-grey, but once I was in box-grey design, a switch to zen had no effect :/ Any idea?