FOUND OUT THE PROBLEM!
There are some settings that I save its state (TRUE or FALSE) inside one array to, depending of what it presents, print different metaboxes in the panel.
The problem is that — I was printing the metaboxes by any means without check if the array was empty or not...
And since they used the very info to show a SELECT with OPTIONS (where the options are directly dependent of my informational array) things got broken.
Woow... What I relief. Minutes after posting the topic I did remember about this scene in my code.
Moderator please, can you lock the topic? All good now.
And FOREVER grateful for you suite of MetaBoxes AWESOMENESS.
Really THANK YOU.