Maybe I did a mistake when migrating the data.
When I try to view the children of any resource, but the root /, it duplicates the /projects string, and this fails.
e.g., instead of .../projects/courses?action=children, it shows .../projects/projects/coureses?action=children
Which fails of course.
Any idea, where the related data is stored in the database, so that I could fix it there.