Thank you for your suggestion to update to 2.9.0beta1.
According to the release notes “2.9.0.beta1 contains no new features or other major changes”. From my understandíng the beta 1 is always a copy of the latest beta of the previous version.
Can someone confirm that there were bug fixes regarding bookmarks in 2.9.0beta1? In that case we will upgrade the system during the next days.