I just noticed strange behavior:
When I click the Hamburger once (and my mouse doesn't show strange behavior otherwise), I'm directly taken to StackExchange, i. e. the drop-down menu doesn't open.
It only opens on the main site and then I can go to another site. Usually, I will use the Hamburger menu to go to the specific meta page (I know that there are other possibilities), but when I'm on that other site and click the Hamburger, I'm back at StackExchange.
I tried stacks where I have an account on and one where I don't. Same behavior each time. The sites seem to work properly, apart from that.
Edit: Recent achievements and messages take me to my Network profile page (reputation tab and messages respectively).