An update on the 403 error: IT looks like my 'Reverse Proxy' rule was incorrect. After a great deal of fiddling about I got past this error. If I disable the Reverse proxy rule, the 403 error comes back.
The reverse proxy rule is the one that actually shifts the URL from the IIS gateway on port 80, to the native cognos URL on port 9300
My latest issue is that the web page just stays blank for a few minutes then finally show a C11 error that says:
Account information Error
Cannot get users account information!
Looking at the network tab in F12 tools, there's a lot of page download activity, downloading the same pages over and over.
I can see I am now getting a 441 error on page
http://server:90/IBMCognos/bi/v1/identityCognos KB indicates 441 is a Kerberos type issue. But I do have singleSignOnOption set so I don't know why this is an issue.
(Note I had to run this on a different port as Cognos 10 is using IBMCognos)