Permission problem when pressing on links

Hi,

Today I updated wordpress from 2.6 to 2.8. Now that WordPress and all the plugins are updated I have problem with the links to pages and posts. The main pages of my site looks the same as before the update. But when pressing a page link of the link of a post I receive:
Forbidden You don’t have permission to access .. on this server.

On the place of the .. (dots) it gives the name of the page /pagename/ or post /year/month/postname/.

I have looked at the rights of the folders and files of wordpress, but I dont see anything strange.

Does anyone have an idea?

Thank you very much,
Michael