Hi, can anyone explain me if this is normal ? TIA!
Its the zoom button (probably also the pin button hidden behind it). Your windows theme is preventing Opus from correctly rendering the buttons.
You can turn them off here:
Ah, thanks a lot, tanis. Thanks for the great and fast respons. 
Just another thing, whenever I doubleclick the titlebar, it shrink up like this
If i then again doubleclick the titlebar, it crashes...
Just to add, maximizing the 'shrinked up' titlebar, also give the same crash.
I can't reproduce the crash here - try the same thing without the theme your using - just go back to XP's default Luna or Classic.
You can turn off this functionality - known as zoom to titlebar - with the bottom two highlighted options in this screenshot:
Using default Windows themes = crash still occurs.
Switching of the "Zoom Listers..." toggles = crash is gone.