Please make sure your Omnisphere installation is up to date. You can check if there are any updates from the start-up screen of Omnisphere or you can click the Spectrasonics logo on the interface of Omnisphere. Make sure everything is up to date, in particular the Soundsources.

If everything is up-to-date try to delete the zmap.index file found below. It’s harmless to remove:

PC: ProgramData/Spectrasonics/STEAM/Omnisphere/Soundsources/User

Mac: ​Macintosh HD/Users/yourname/Library/Application Support/Spectrasonics/STEAM/Omnisphere/Soundsources

Restart your DAW and insert Omnisphere 2 again and it will automatically create a new zmap.index file.