[Configure category limitations in type form] Invalid field: Category

I have create a new category based on a specific content type...
We created several content items (more than 100) and when trying to update a content item we get the error "Invalid field: Category"
another strange behavior is that the category value is empty and the selection show only one category - the correct one - BUT it is not selectable!!!

Hello

and the selection show only one category - the correct one - BUT it is not selectable!!!

yes,

you have limited the ITEM type to the SUB-Tree of specific category
but by default the spefic category (=root category of the sub-tree) is not allowed (show disabled)

-- either create sub-categories (they will be selectable)
-- or go to item type configuration again and allow this specific category too (=root category of the sub-tree)