Hi Pascal,
A few questions come to mind about this, What version of BP are you running and do you have Multi-Team environments configured? At first glance it sounds like there's some kind of conflict between permissions and configuration of your schedules that is preventing a more administrative approach to executing your schedules. Are you getting any additional failure/error messages that you might be able to share when this executes?
Ideally, the schedule/users should have access to the appropriate processes, and based off what you've described, it sounds like you proceed through a series of actions from "Login as user X, wait, Perform Process X, Log out, Wait, Login as user Y, wait, Perform Process Y, Log Out, wait, ... N+1"
------------------------------
Charles Trott
Senior Product Consultant
Blue Prism
------------------------------