[Jörg Isermann]

Had the same problem.
Red Giant trapcode, Video Copilot Sabre and a codec were randomly not loaded.
Sometimes all the Trapcode family not, sometimes all videocopilot, sometimes nothing.
Restarted hundred of times. Read these and that about pressing Shift while loading. Reinstalled Plug Ins as Administrator and all this waste of time.

Finally, I switched to 2018 today.
It seems to work, but no promisses, no guarantee, as I read about other experiences, which could hit me in future as well.

What an annoying unprofessional time killer no one feels responsible for.
“Contact third party plug in vendor – contact adobe” tennis
I really avoid this this time as it would even dissapoint more.

Good luck! Help yourself and pray an upgrade will help.
Read about other stepping back to 2014, but I guess this is not an option for you?