Exception occurred calling method NotesAgent.runWithDocumentContext(lotus.domino.local.Document) null

Tried to run agent with 'context document' in xPage (via SSJS) and got this problem. I did not find a way how to solve that except to enable checkbox 'Run as Web User' for agent, however I need to run agent from 'signer' as 'runner' has Reader access and my agent update should document. It's terrible with this problem (if it is problem ofc :)). What should I do know?