Cubase 6.5.5 on OS X Yosemite

After upgrading 10.9.5 Mavericks to Yosemite everything worked fine, including Cubase. I was able to load the program (the license key had not been updated at this point), load a song and play it. This was just once, since then Cubase crashes every time I start it and load a song. I have narrowed down the problem to a VST3 plug-in:
Cubase 6.app > Contents > VST3 > Cubase Plug-in Set.vst3

As soon as I remove that plug in the application loads, I am able to load a song and the app does not crash. Of course I get 300 errors because the plug-in is missing.

First question: are there updates I can perform on this particular plug-in set that could resolve this issue?
Second question: If not, can I downgrade to 10.9.5 Mavericks and will the license key (that I updated to see if it would resolve the problem) still work?

That was a dangerous move, since not even 7.5.30 is officially safe with Yosemite. I don’t know anything about Mac, but your license is okay. It resides in your USB-key and won’t be changed by any software change. The e-Licenser software is just the go-between with the license and the software needing it (Cubase).

Thanks Arjan P.