Hey Karsten,
first of all thanks for your continuous effort in the community!
I’m right now struggling with kind of the same problem and noticed some weird behaviour, for that I currently don’t have a solution yet:
In case of an open task that is overdue, the “next activity date” is empty.
This might be logical, since the date is in the past and not upcoming.
However it leads to problems, since it seems really difficult to construct a filter, that includes only deals, that have no or no open task.
What makes the situation more complicated is, that most of the deals already have a bunch of activities as well as some closed tasks as well.
The ultimate goal is to build a workflow, that creates activities for all deals, that are not closed and have no open task.