<<Having looked a bit more at categories, I encountered another strange phenomenon. On Opel Laubfrosch https://en.wikipedia.org/wiki/Opel_Laubfrosch, which has a lot of categories, the order of the cats on the page settings tab changes. Initially I get 8 categories, from "Opel vehicles" to "compact cars". But when I e.g. click on the drop down button next to 1920s automobiles, and then click again in the "categories" window (to close the drop down without any changes), I get 10 cats instead of 8, and in a different order. "Vehicles introduced in 1924" and "Sedans" are doubled (explaining the increase from 8 to 10), and the 3rd one is now no longer "1920s automobiles" but "1930s automobiles"... This can go on for quite a while, with the occasional disappearance of a cat as well apparently (normally the last one). Things start to break down, with the page settings then no longer working, or it being no longer possible to add a cat, and so on.>>
I can confirm what he says, and this bug does not look browser or skin specific.
VE will let you save in a situation where you have these cats duplicated, and a "saved edit" message will also pop-up, but there will be no trace of the edit in the history. Plain removal of all the categories works fine instead: https://en.wikipedia.org/w/index.php?title=User%3AElitre_%28WMF%29%2Fsandbox&diff=576123603&oldid=576122965 .