The author was probably over enthusiastic about his achievements. An honest programmer would have put out the upgrade as a 2alpha0 version, not a ..5
Luckily he does not update his blog, so I could get the "old" (i.e. fully functional) 1.2.0 from there to roll back.
So WATCH IT, this plugin might be great (that's what I thought until a few minutes ago), but the programmer does not really care about upgrading an existing install, it seems.