Add Portal Failure in VertiGIS Studio Analytics

Hi all,

I've successfully installed on-prem Analytics 1.5.1 and set up agents etc. The agent is happily monitoring our ArcGIS Portal server. Set up using IWA with anonymous access disabled.

However, when adding a Portal as a resource - I'm getting authentication issues, Portal oAuth page loads but can authorise:

Anyone experienced this or have any ideas?

Cheers,
Callan