Third level of categories not displayed in tree

-- HikaShop version -- : 2.6.4
-- Joomla version -- : 3.6.4
-- PHP version -- : 5.6

Hi,
When i look at my category tree there is a + on the second level, but when i click on it the categories don't show up.

I tried to rebuild the categories, but this also don't work.
In the categories-view it's not a problem because you can click on the categoryname, but the tree also don't work in the modules, menu-settings etc.

as there are 277 categories in here, i dont like to start over again
is there a way to fix it? i can send you the database-login if you like.

THANKS.

Have a nice sunday Nicolas!