For vCO Task Scheduler I have a stupid fix... until a better solution will be available:
- leave an open session of orchestrator client
- delete old schedule task and create a new one with same configuration and same workflows
In that way seems like userToken is kept as long as vCO Client is connected to server.
It's working on vCO 5.1.0 (windows setup).
If you still want to upgrade to vCenter Orchestrator 5.1.0a or 5.1.0b or 5.1.U1a (Windows Setup / Not Virtual Appliance) you can find the last versions of windows binaries inside VMware vCenter Server ISO files (VMware-VIMSetup-all-5.1.0-*.iso).
Hope this helps.