Omnisphere 2.5 and above require a 64-bit host. If you are running a 32-bit host, or 32-bit operating system, Omnisphere 2.5 and above will not work.  

If you are running a 64-bit host, make sure that you run all of the updates from your Spectrasonics.net 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.

Here is the default 64-bit plug-in location for Spectrasonics Instruments on Windows:
 
C:Program FilesSteinbergVstplugins

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 from Legacy Updates.

Next, please delete the following files: 
 
Windows
 
C:/ProgramData/Spectrasonics/STEAM/Omnisphere/Settings Library/Patches/Factory/Hardware Library.db  
 
C:/ProgramData/Spectrasonics/STEAM/Omnisphere/Settings Library/Presets/Factory/Hardware Profiles.db  

Note: ProgramData is normally a hidden folder. To "Show Hidden Files" go to Start Menu/Control Panel/Appearance And Personalization/Folder Options/View tab. Select the "Show Hidden Files and Folders" option and hit "Apply”.
 
Mac
 
Macintosh HD/ Users/ / Library/ Application Support/ Spectrasonics/ STEAM/ Omnisphere/ Settings Library/ Patches /Factory/ Hardware Library.db  
 
Macintosh HD/ Users/ / Library/ Application Support/ Spectrasonics/ STEAM/ Omnisphere/ Settings Library/ Presets /Factory/ Hardware Profiles.db  
 
Note: To access this hidden Library click Go from the main apple menu at the top of the screen. 
When the go menu is visible hold the Option key - Library will appear and you can select Library at that point.