I have finally found that there was a firewall blocking the DNS request from the Unity VSA that was causing that error.
We have install the unityvsa and everything was working correctly. Last week they asked me to change the subnet of the management interface.  Since that date I cannot connect using the FQDN anymore I'm always getting a 403 message :   When using the ip the GUI is opening correctly. I remember that the first time we installed it we had that error message until I ran the initial config wizard.   How can I fix the configuration now ?  Thanks  Luc Bussières
GUI access not working by FQDN just by IP