Summary
Some proxied web sites receive the error "Invalid server response received, your session may have time out." when both Access Gateway nodes are enabled. The proxied web sites work correctly when only one cluster node is in use.

Products
Access Manager (NAM)

Environment
Access Manager 4.5
Two node Access Gateway cluster

Situation
When accessing some websites via an Access Manager reverse proxy, authentication is successful but the browser receives this pop up message
Invalid server response received, your session may have time out.
The following message was in the F12 browser trace.
Cross-Origin Request blocked: The Same Origin Policy disallows reading the remote resource at https://<idp base url>/nidp/idff/sso?RequestID=idxxxxxxx etc …. . Reason: CORS header ‘Access-Control-Allow-Origin’ missing). Status code: 200

If one Access Gateway node was shut down, no errors were seen and access to the proxied web site worked normally.

Read Full Knowledge Base Article for Cause and Resolution. 

URL Name
KM000012698