A client is attempting to continue a pending sign-in request, but the target of the continuing request differs from the target ...

A client is attempting to continue a pending sign-in request, but the target of the continuing request differs from the target of the pending request. Each browser session may only maintain one pending request at a time. 
Continuing request target: %1 
Pending request target: %2 

This situation may occur if two Microsoft Office applications attempt to perform AD FS authentication simultaneously because session cookies are shared across all Office applications. 

User Action 
There is typically no action to be taken at the server to correct this situation. A user will see a failure in one of the Office applications. Reopening the failing Office file after the second file finishes loading resolves this issue.