Omnisphere 2.6 requires a 64 bit host. If you are running a 32 bit host, or 32 bit operating system, Omnisphere 2.6 will not work. If you are running a 64 bit host, make sure that you run all of the updates from your User Account (Software, Patch Library, and Soundsource Library), and if using Windows, you are selecting the correct location for the updated .dll to be installed.
If you are not using a 64 bit host, you can revert to a previous software version by going to the Legacy Download Area at the bottom of the Omnisphere 2 Updates page. To revert back to Omnisphere 2.4 which supports 32 bit, install Software 2.4.2c, Patch Library 2.4.0d, and Soundsource Library 2.0.1c.
Lastly, you'll need remove the Hardware Library.db file located here:
Macintosh HD/Users/Username/Library/Application Support/Spectrasonics/STEAM/Omnisphere/ Settings Library/ Patches/ Factory
C:/Program Data/ Spectrasonics/ STEAM/ Omnisphere/ Settings Library/ Patches/ Factory