Finally, after 6 months of waiting I have had the issue fixed regarding Dolby Vision from external sources (eg. Apple TV 4K). B&O came to my home and changed some of the eletronics in the soundbar, the cable between soundbar and TV panel and also new software.
The service guy fra B&0 said, it had been extreemly complex to indetify the problem and how fix it. He told me, that B&O was still searching for a way to fix the problem with software alone, in order to avoid a major repair operation. He did not know if they would successfull or not.