Mathias,
In the spirit of WasabiNME's post here
http://forum.multicommander.com/forum/index.php/topic,1774.0.html, this has to be, I believe, a bug or at least an oversight.
It is a folder size issue: if
any file/folder is selected in the active tab then calculation of folder sizes - using shortcut Shift+Alt+Enter
or Menu->Tools->Calculate folder size - fails. If this were changed, it could, for example, allow users to determine if they can copy/move the selected files/folders to the target.
Can you please take a look and either add it to the next release, or explain why it would not be practical.
TIA
Alan