It could be a couple of other things.
In RSReportServer.config, locate <UrlRoot>. Does it start with HTTPS and does it match the domain name shown in your certificate?
In RSWebApplication.config, locate <ReportServerUrl>. Does it start with HTTPS and does it match the domain name shown in your certificate?
If both of these are okay, also check your registry for the dword DisableLoopbackCheck located in HKEY_Local_Machine\System\CurrentControlSet\Control\Lsa. If it doesn't exist, try creating it and then rebooting the server. I've seen this be needed when integrating SSRS 08 with SharePoint so it may be applicable here as well.