Thank you so much Joachim for taking care!
When I altered my CPTs to hierarchical=false the problem was gone there.
To get rid of it with my Pages too, I deactivated all other plugins, but the problem was still there.
So I reduced my code base ... (and reduced and reduced and reduced)
At some point nothing was left.
So I started an experiment: I have now 2 new projects with exactly the same files and exactly the same configuration. Just the basic files and the CAS-Plugin, no functions.php, no sidebars, no widgets, no permalink-altering, nothing special.
One project does show the database error on creating new Pages, the other does not.
Is it possible, that your plugin has written some resistent configuration data at some point? Is there a place to look for?