We had an issue recently when upgrading a UAG server to SP3.
The upgrade ran perfectly with no errors , but when it came to us browsing to the UAG portal externally we got the following error “Forbidden Directory, Listing Denied Error code 403.14”.
To Resolve this we had to do the following:
- Open the UAG Management Console
- Disable all Trunks
- Activate the Config
- Enable all Trunks
- Activate Config
- Run an IIS Reset
After we did the above all started working fine 
