If you guys wish to know what happened.
I tryed to install a quite simple Country Flag mod, fully compatible with the actual SMF version we got, right, checker told me everything would have been fine and the mod wouldn't do any harm.
Installed the mod, I added my flag.
1 minute later, BAM, sorry, Fatal Error.
I'll look what went wrong after, got a small idea though... Need to check it at another SMF forum (time to build one).