@dajo
Thank you for your reply.
Yes the url's are created by the Kunena extension.
I did not update the Kunena component itself.
But, in the Joomsef admin an available update was shown for the Joomsef Kunena extension and I did the update.(to version 2.1.4)
Current Joomsef Kunena extension status:
Kunena Forum Kunena Forum ARTIO s.r.o. 2.1.4 1. February 2012 2.1.4 Free Up to date JoomSEF extension

Sorry, I did not mention Kunena version. It's Kunena 1.7.1

Please open the Kunena extension parameters and post a screenshot of your configuration for the Extension tab. Thank you.

Here is the screenshot. Thanks for your help.

Try setting the "Category name 2nd part" to "Title" instead of "Custom text".

Thank you for your help.

I've changed the category name 2nd part as advised and the url looks better. The double forum/forum/ is no longer in the URL.
Now the URL looks like that :
www.avi-plus.com/categories/repair-tips-forum/page-14.html#1295
I'm trying to replace the /category/ with /repair-tips/forum but to no avail up to now.
The URL should look like this:
www.avi-plus.com/repair-tips-forum/repair-tips-forum/page-14.html#1295

I had also to disable the Joomsef cache in the configuration panel as the forum page generation was completely wrong for new pages.
If you clicked on page 14 on the topic, the forum was showing 27 pages i.o 14 and the user was never able to see the real page 14.
Now with the cache disabled, it's fixed.
In fact I have disabled every cache = Server cache, System cache, Joomsef cache.

Sorry for the mistakes in the above url's

They should look like this:
www.avi-plus.com/repair-tips-forum/misce...rfacing/page-11.html
But actually the new generated URL's ar looking like that:
www.avi-plus.com/categories/st40-stb71xx...rfacing/page-12.html

Does that mean that all settings are reset when doing an extension update ?
Thanks for your help. (Still trying to fix that issue)