When trying to create or edit posts in wordpress, the error message “You are not allowed to edit this post” appears. As mentioned, my user is an administrator. With a different administrator account I still have the same problem.
I first thought the problem might be caused by my wordpress template, so I tried to change it back to one of the default templates but the issues unfortunately still exists.
The second step was to download wordpress, unpack it and replace the wp-config.php with my own wp-config.php to give the new installation access to my old database. After that the problem still existed!
After that I came to the conclusion that the problem can only be inside the database, right? 
This is a screenshot of SequelPro looking at the wp_usermeta table of my wordpress database. Maybe someone knows what could be wrong with this? Where else could there be some misconfigurations that causes this problem?
Thank you! 🙂
Yours Sincerely, Felix
I found out that the problem was that somehow there were no primary keys in my tables. That caused all the trouble I had :/