Please close this issue, it was a server problem, not a moodle problem.
Following situation: A User logged in in moodle2.example.org from moodle1.example.org via mnet, who is site manager on moodle2 and subscribed to the course as teacher, can not edit the first page of a wiki. Error: Forbidden You don't have permission to access /mod/wiki/edit.php on this server.
This is only for the whole wiki page. If I edit a special section, it works, no error. Wiki setting is forced creole formatting.
mod/wiki/edit.php?pageid=15§ion=Inhaltliche+Bereiche WORKS FINE