I just tried to setup multiple inboxes as described in this Google video.
Unfortunately I notice that the "Save changes" ("Änderungen speichern") button is greyed out!
Maybe my filter-rule is wrong? I took a screenshot of the example setting that I tried:
I know that before I once had "multiple inboxes" in use, so I'm really confused as to why I suddenly cannot setup those anymore. As the tab shows up, the labs feature is obviously activated.