This issue is caused by a missing registry key. Opening the Galaxy Client and using the Verify / Repair function from MORE > Manage installation > Verify / Repair should fix it.
Alternatively download the beth_registry.zip archive from here:
Copy the beth_registry.cmd file from inside the archive to the game's directory, default:
C:\Program Files (x86)\GalaxyClient\Games\Oblivion
Right-click the beth_registry.cmd, select to "Run as administrator" and chose to update the registry.