Hi Hajo,
Because your question has had some answers added but not totally solving your problem, it may be prudent to resubmit your problem so that it rises to the top of the list for others with a greater knowledge base than me to help you.
By the way, having seen a newer problem than yours in this forum, version 11.26 is still in Beta format. Since its release, there are some known bugs. Beta versions are used by the long term experienced X -Plane users specifically looking for bugs to assist Laminar Research in developing the software. If you are not into "bug finding" I would suggest you don't "tick" the option to update to a Beta version when the message shows on your screen that a new version is available.
Best of luck.
Glenn