An update
Since seemingly no one has no clue from the problem I’m describing, here is an update.
Firstly on the lost bookmarks: I uninstalled Opera from GooglePlay – NOT from the Android settings where one can delete all cache and data – and reinstalled it. All Bookmarks were back. What a great relief…
Secondly on the blank screen: I tried the full screen mode and suddenly I could read, zoom in & out etc the old fashioned way without the previously described problems. When I exited the full screen mode all hell broke loose again. Which means this is definitely a software bug, and it would be very kind of the programmers to address this problem.