alltough our board filters, status and workflows are correctly configered.
Solution:
At the end we had to recrute an external consulter and found the problem, it was an difault obstacle hard to find. due to permanent updates of our jira system, as we do have the cloud version, the new projects had a totaly useless extra defoult filter for scrum sprints integrated in the default kanban boards. a filter by "version"/ as we don't use versions, non of our issues had one and therefor non issue was shown in the board. some extremly counterproductive default setting change which costed me now one week of work and money for en external consulter...