Design History - Combine Button greyed out, although MS_DESIGN_HISTORY=combine=1

Hello everybody,

the menu button for the combine command is greyed out, although the variable in the confiig file is set to 1. And when I use the key in: HISTORYMANAGEGUICOMBINE the tool pops up.

Has somebody experienced this problem? Am I missing a config variable, or another option to get the button to work? 

Thanks in advance.