I've seen several problems when creating the structure.
1. It's confusing, there are several apparent ways to add main and sub categories.
2. Using the "Add new" button at the top:
I entered the first category. I can then click on "Add new" again at the top, to enter a new one. It actually discards my current category without any warning. One has to click on "Publish" to save it first.
If I click on "Add new" and there is nothing new to save, I sometimes have the "Leave site?" warning as if I were about to lose data.
3."Save forum order and hierarchy"
If I click on this button after changing the order, I get the "Leave site?" warning as if I were about to lose data, when I'm explicitly asking to save them!
4. Moving items in the hierarchy.
It's not possible to move an item under a same category and change its place. Moving it will only allow to place it under another another item which was previously at the same level. So if under "A", I have sub-categories "1", "3" and "2", if I move "2" after "1", it will actually put it as sub-category of "1".
Since creating items put them in the wrong order (last first), this is very cumbersome.
There are other issues but I'll stop there for now. I must say I'm hesitant to continue...
Before doing all that, press F12 and watch your browser CONSOLE for errors. That could be it.
An other possibility is plugin conflicts. You can check for those by using Standard Wordpress debug to a log. And check the log.
I have no error with the "Add new" not registering my current entry and not giving any warning.
When I click on publish to register the new entry, then on "Save forums order and heriarchy", I get the warning (leave?). I can't open the console before confirming the pop-up, but if I open it after, I see 1 error:
admin.php?page=wpforo-forums:349 Uncaught ReferenceError: navMenuL10n is not defined
I could not enter text after the quote in my previous post, so I'm continuing here.
I had other issues (here), which are more concerning, so I'll probably wait to see if that can be solved before trying anything else. There is apparently a conflict between WordPress and wpForo in what we can choose with the permalinks. Maybe that's just my server config, but it's quite standard.