I've the same problem in OS X Yosemite. During the scan process S1 stops at Steinberg plugins (it seems to stop scanning Groove Agent or Retrologue, I don't know exactly). I've deleted these plug ins in the VST3 dir but the result is always the same.
I use many sequencers (Ableton Live, Cubase Pro, Reason) and I'd like to use also S1 but if this is the beginning, I've no time to waste and I'll return the product to Presonus !
Please Presonus, give a solution to this problem, because I cannot use your product.