I can try 
to translate a short version:
We do have an open time position for a project because we forgot to bill that time position with 0.25 hours. The problem: the employee does not work for us anymore and the booking period is closed. So we looked for an answer to clear that position in ByD, because we do not wanted to have open projects and time positions that are not "finished".
It was only possible to re-open the booking period as Wolfgang Walz has suggested and to delete that time position. This enabled us to close the project properly.