Hello Theodoridis Christos,
Thanks for your post in Microsoft community. This is known issue and our related team is working on this issue. You can check it via Screen Sharing with 3440 x 1440 Resolution shows as a black screen. Meantime as a workaround, they suggested to please try using either ‘application sharing’, or change your resolution to a higher or lower resolution for screen sharing and seems at this moment you are follow the same workaround. So once the fix will be available, we will post back here and you can also observe the current status of this issue by observe the above mention link.
Your kind understanding and co-operation will be highly appreciated.
Thanks and regards,
Ankita Vaidya