Ah, I feel stupid now, I somehow managed to completely miss out on that step -.- But even still, appending index.php/upgrade doesn't seem to work for me, I get a "Page Not Found" error message. (Unless this is expected ...

In order to properly upgrade, you would have to also drop the "Activity" table in the database and run the following SQL through phpMyAdmin (change et_activity if you're using a different prefix):

CREATE TABLE IF NOT ...