New/Edit post – No categories showing up

Since migrating to 2.5.1 from 2.5.0, I have a problem with my categories.

When I write a new item, the categories do not show up. I can click “most used” and all of my categories will be in there. However, even when I click one of those categories and save, it somehow reverts to the default category.

Likewise, when editing an existing item, I do not see a list of categories. They do show up under “most used”, however the previously selected category/categories aren’t ticked. Changing the category or ticking an additional category does nothing: the category selection remains unchanged when saved.

Thanks for your input.