The ispconfig part is fine, the problem is wordpress here,
I guess that you had this wordpressinstance installed before, so its a migrated website or at least some of the wordpressdatabase contents are from a existing install. Wordpress saves paths in its database and the wrong path that you see in the error.log is set inside the wordpress database. You can search the wrong path with e.g. phpmyadmin and ajust it in the wordpess db.