Hello Malton,
Welcome to VMware Communities.
Error saving Vcenter Entries is pretty generic and I would say look for the Account used while adding Vcenter.
Try to login to vcenter using the same credentials that you are using for adding the Vcenter in horizon and make sure you can login successfully.
Apart from that if you would look for logs the easiest method would be
Rep-roduce the issue: open the debug file in the connection server logs( DriveLetter:ProgramData\VMware\VDM\logs) and search for https://vcenter.domain.com/
Check Windows Event Viewer logs in the View Composer machine