If this is still happening to you, could you please let me know the following information:

1. Which specific categories this happens to (and which specific gallery is the category in)
2. Which item(s) did you attempt to move into the category
3. Does it seem to happen with ALL of your categories?

Okay, I figured out why it's happening to your specific gallery (but it's still a glitch on our end, sort of)

Could you try going to the "Edit Categories" page of your gallery and deleting the extra space in between the words in each category name, save changes, and see if that resolves the issue?