I'm sorry, I was too brief - I had explained my problems elsewhere.
1: No more automatic install:
The option to do an automatic update is still present. That directs me to the page 'Connection Information', where I enter my FTP / SSh password. I immediately get redirected to an empty admin page, and nothing happens.
2: Impossible to add tags in the New Post screen:
Yes, 'post tags' is checked. I see the box for adding tags too; but everything that I enter is greyed out, and the link 'Choose from the most used tags' doesn't do anything anymore. However, in 'Quick Edit' mode I can add tags.
3. Error with spam posts:
On the 'Comments' page, I get the aforementioned error as soon as I select to see 'Trash'. The error also occurs after the command 'Empty spam' has been processed.