I finally managed to unlock my access to my WordPress blog. Since the last upgrade I always received the “You do not have sufficient permissions to access this page error”
I installed a fresh WordPress (new empty database, new path). Once the setup was done I copied everything from the old database to the new database except for wp_user and wp_usermeta. After logging in as an administrator everything was working as before the error. I created new users for posting content and so far I’ve not encountered any errors. I still have no clue why this error suddenly popped up.