At the beginning of the new year Veeam has built the Release Candidate for Veeam Backup and Replication 9.5 Update 1.
With this update it is possible to backup VMs running on vSphere 6.5. You have to open a case at Veeam to get the Release Candidate. Remeber: Those updates are only for testing. Productive support will come with the official update!
Open a case at https://www.veeam.com/support.html?ad=menu-support
You will get an email with an FTP account to download the ZIP file.
Installation was easy. Extract ZIP archive and execute the EXE. After installation the build is now 18.104.22.1682
All Backup Jobs are running now without any problem.