This may be a permission conflict with the folder containing the REX Shared Library:
- Locate the folder /Library/Application Support/Propellerhead Software/
- Right-click it and select Get info
- Go to Sharing & Permissions
- Unlock the option on the bottom-right corner - you will be asked for your admin password
- Ensure System is set to Read&Write
- Click on the cog wheel icon (under users list)
- Select Apply to enclosed assets then validate