Hello Joe,
thank you for all your help! I have now identified the problem and made corrections accordingly. For some reason two sales orders had mismatch between quantity ordered versus quantity delivered. The only mystery is that I can't find any delivery notes showing that delivery was done double times or so. As long as there is no delivery note showing that quantities are exceeded, I cannot point out the real reason why this happened (e.g. human factor). For the first mentioned sales order I remember that we delivered half of the items in a first shipment. After that we slightly modified the sales order and then delivered the rest. Maybe this has led to this situation - cannot say...
Anyway, I directly edited the database from table 'sales_order_details' and now inventory seems to be in balance again.
.:Solved.
Thanx, Mika