I just noticed that my German vocabulary is being listed as 0 both on my homepage and profile. Just a heads up in case this is a widespread glitch!
I have seen something like that happening when I have one tab open in one language, and one in another. Duoligo server records the latest state, ie. the last language you swiched to (french or italian in my case), so if I have a tab open in french, open a new tab, where I switch to italian to do some work, and then go to the first tab without switching back to french, then the links work, and it opens the pages from the french tree but I see zero vocabulary in the units. The solution for my case is to switch to french again, and then everything works.