I suspect this has something to do with recent changes at the Facebook end. At least we encounter all kinds of (un-announced) changes/odd behavior with respect to live streaming. Compare a discussion in Streaming idiots about where to find the preview nowadays.

In any case, when we try to login to Facebook via vMix Social (5.0.0.12) all things seems to be fine at first. Checkbox marked under Password (keep me logged in, or whatever the English version says). There is a warning that one should update the browser (I guess Facebook means whatever vMix Social interacts through). The login procedure sort of ends with OK from Facebook's side. (Can't replicate it right now cause I have made too many attempts recently).

But back at vMix Social I am still not logged in...

Tested with multiple Facebook accounts on different PC:s and browsers.