Does the forum logs give an error when it crashes?
Do you have mongo and nginx logs on?
There should be some useful information in one of the logs as to what is happening.
I don't know if it is a already known bug.
I made a subcategory with the name "Urlaub" with the subsubcategory "Winterurlaub" (both contain the string urlaub), resulting in a behavior that the icon for both are the same ( in my example I had no Icon). Also a picture for one of them couldn't be saved. Disapears every time after uploading. After changing the subsubcategory to "Winterlaub", both pictures and icons can be setup for themselfes.
The save button on each category is actually a global save button, the only reason we added it to everyone is so you don't have to scroll all the way to the top or bottom to save when you have a lot of categories.
You can test it by making changes on two categories. When you click save on one of them the success alert should show the category ids that are saved.