I had a similar issue (all VST libraries working in GA5 after new PC/fresh installs, etc, except Songwriter Essentials). Tried everything but in the end, this worked for me (damn firewall!!!):
Cheers!
- Uninstall VST library from Steinberg Library Manager
- Uninstall VST library from PC using Add/Remove Programs
- Disable all firewall/virus protection
- DL/Unzip/Install library in default folder
- Re-enable firewall/virus protection