EXACT STEPS LEADING TO PROBLEM: 
1. edit the main page: add an internal ling beginning with a "/". E.g.:
[[/subpage]]
2. click onto the red link to enter and edit "/subpage"
3. write somthing and save

EXPECTED OUTCOME: 
The page contains the stuff written in step 3. and on top of the page there is
a link back to the main page. A breadcrumb. 

ACTUAL OUTCOME: 


Here is a description how to enable this in media wiki:
(from
http://www.gossamer-threads.com/lists/maemo/community/48640?search_string=breadcrumbs;#48640
)
---
In LocalSettings.php:

# Enable subpages in all namespaces
$wgNamespacesWithSubpages = array_fill(0, 200, true);

and in the skin, I have:

<div id="breadcrumb"><?php $this->html('subtitle') ?></div>
---

I did it several times in media wikis I'm administrating and it works fine.