Weird.
I've tried to replicate the problem but nothing I do makes it happen.
Typing '.5"' (without the outside quotes) as you have said just makes Scribus revert back to what was in the field originally - the " quote character isn't recognised as a digit and Scribus ignores the input.
Typing '.5' (again, without the outside quotes) sets the field correctly to 0.5000 inches (when inches are the unit type that is set).
Typing '0,5' gives a value of 5 - Scribus ignores the '0,' portion of the input.
Typing some random letters reverts the value back to what it was before (that was just to see what happened).
I just can't get it to do what it's doing for you.
If you can give your OS version and Scribus version (best to copy the info from the About Scribus dialog) then we might be able to look at what's happening.
If your video is smaller than 2MB then you can put it in a ZIP and attach the ZIP to a post. Otherwise, a link to DropBox/Google Drive/whatever might be a good choice.