Error: Cannot open Multi .../default.mlt_omn?

This means Omnisphere could not find a correct STEAM folder. When completely installed, the Omnisphere folder inside your STEAM folder should have a total size in excess of  42GB. If you have completely installed Omnisphere, but the problem persists, that indicates the alias or shortcut to the STEAM folder in your Spectrasonics folder is incorrect.

To fix this:

Follow the instructions in THIS FAQ to locate your Spectrasonics folder.

1. Go to the Spectrasonics folder and delete the STEAM alias (Mac) or STEAM shortcut (Windows) you find there.

2. Load Omnisphere and follow the instructions when it prompts you to find the correct location of your STEAM folder.