Found out the problem!
I had in my functions.php an old piece of code that was loading a old version of SELECT2 to be used inside ADMIN area for other purposes in the past. Forgot to remove it, so it was conflicting with SELECT2 used on the METABOX plugin.
I simply removed my manual SELECT2 script enqueue and everything is fine now.
So, if you come across a error like that, first check if you don't have another call for SELECT2 being enqueued on your ADMIN area via ADD_ACTION('admin_enqueue_scripts', ...)!
Guys, please mark as solved!
Thanks! Regards,
G.