(Sorry, my German is good enough to read your suggestion, but not to reply in your own language.) Check the relationship behaviors between the Contact entity and each type of activity. (zB look at the 1:N relationship from Contact:Phone Call, Contact:Task, usw) It looks like you have this set to the default "Parental" template, so when you re-assign the owner of the Contact it also reassigns the owner on all the activities. Because this reassign changed the activities, it changes the "Modified On" date. Unfortunately, this date is used for sorting activities in the social pane, rather than the planned end / actual end. I would recommend you change the behavior to Configurable Cascading and then change the Assign action to "Cascade Active", so that completed activities are unchanged while open ones are reassigned (or change it to None if that fits your business processes better).