Symptoms

I want to start a K2 Workflow via Sharepoint retention policy.

The Workflow is bound to a Site-Content-Type via Sharepoint Integration wizard.
If I start the workflow via WebInterface => everything is fine
If I start the workflow via SharepointShell (Workflowmanager) => everything is fine.
If I start the workflow via Information Management Policy, Sharepoint tries to start the workflow, but is does not succeed, Sharepoint says:
"An instance of the TMD.HammDocCenterIMP-Reminder workflow could not be started. Check the event viewer for more details. - Workflow started, but with errors"
"The workflow has been completed. - Workflow completed."
No instance is created in K2.

It works on our DEV-Server, but not on our production system.
 

Diagnoses

My diagnosis was still heading into the investigation of permissions of all applicable accounts
 

Resolution

the user is running the AppPool, so that user has every right in every site (implicit). When the client logged on as this user and tried to start the workflow via GUI, everything went fine - but not via Information Management Policy.

What solved the issue: We gave the user explicit permissions in the site collection. After that, the IMP WorkflowStart worked. We removed the explicit rights afterwards -> still working.