Wiki pages with same name URL broken

Strange thing: After creating many pages in the Wiki where we described different departments we also created a special wiki with all the different tasks for each department.

So we have the following for our descritpion of the backoffice department (was created first)
domain.com/wiki/backoffice

And just now we created the SLA page for that department:
domain.com/wiki/sla/backoffice

When we now click on the toplink we go to the bottomlink and we see that page, with in the bottom the following error:
Warning: Invalid argument supplied for foreach() in /home/deb44105/domains/4laun.ch/public_html/wp-content/plugins/wiki/wiki-include.php on line 755

Hope you can help us soon because we run this in a production environment.