When in the configuration "url-rewriting" is set to ".htaccess", then the link for the images is not correct:
wrong: /wiki/_media/playground/test.jpg?w=&h=&cache=cache
correct it should be: /wiki/data/media/playground/test.jpg?w=&h=&cache=cache
The effect is that images are not shown in preview and after saving a file.
More info:
http://forum.dokuwiki.org/thread/2590
Thank you for your help!
Markus