I continued to play around and it has gotten better.
What I did was disable all plugins using https://codex.wordpress.org/FAQ_Troubleshooting#How_to_deactivate_all_plugins_when_not_able_to_access_the_administrative_menus.3F
after that the wp-admin told me I should upgrade jetpack. I did. At the end it sent me a blank page with status 200 ok - which i don’t get at all - anyway after that the wp-admin said everything was up to date.
The wp-admin/plugins.php works again. I disabled all jetpack functions and the plugin altogether and now i think i can edit posts (wp-admin/post.php?post=8948&action=edit) - although I still don’t get a list of posts in wp-admin/edit.php