I've been dealing with this issue for well over a year now. It all started when I did an ArcGIS Enterprise migration from on-premise to Azure. During the installation and migration I was connected to my agency's VPN. Ever since then, If I copy the URL of a service in ArcGIS Enterprise and try to add it as an item in AGOL, it fails unless I'm connected to VPN first. Once the service is added, it works fine even off VPN. However this process has become annoying. My IT Department has determined that there is nothing in our firewall or in Azure blocking Enterprise and AGOL from talking . At this point we feel that while on VPN during the migration, the VPN's IP Address has somehow embedded itself into ArcGIS Enterprise.
The only fix I know to try is doing a complete uninstall and reinstall of ArcGIS Enterprise off of VPN.
Has anyone else ever had this issue?
Thanks,
Michael