Categories drop-down empty after migration to 1.16.x

[KnickKnack]

Hi Community,

I've recently upgraded from 1.12.x to 1.16.x and I've done it first on a testing environment and, upon success, on production environment.

Today, after a few days from the upgrade, I've realized that on the production instance the list of categories from category-drop down menu, is completely missing either on front-end (typical case when you want to move a thread) and on back-end (typical case when you want to set privileges).

Production instance(left) Vs Test instance (Right)

I've no clue what could be the case and how to get it fixed.

Thanks, Riccardo