Hi Fabio,
Thanks for reaching out with your BoldGrid Post and Page Builder questions!
I tested your issue in Firefox and was unable to replicate the error. In my instance the Post saved properly as it is supposed to. Have you tried this out in any other browsers?
Is your WordPress core/theme and Post and Page Builder plugin updated to its latest versions? Also do you remember when you first started experiencing this problem? Was it maybe after an update of some sort, or after installing a new plugin?
We look forward to assisting you further with this Fabio!