Ok, the problem is fixed. I've restarted everything from scratch like this :
Remove the Joomsef plugin (with its Kunena extension)
Reinstall Joomsef, last version, and reactivate the paid version with the download ID.
Insure that the Joomsef plugin is enabled.(Joomla plugin management menu)
Here are details on the Joomsef Kunena settings. (hope it will help others, as it took me a while to resolve that issue)

How I've setup the Artio Joomsef Kunena extension (SEO) when Kunena is the unique and main entry
page in Joomla 1.5.26

In my config, I've enabled only the Joomla TOP menu with 3 menu items.
The Joomla main entry page is the Kunena forum index and is named "repair tips forum".
This allows the following friendly URL generation :

when the Topic name 1st part is set to Title in the Joomsef Kunena extension config panel.(Extension tab)

Topic name 2nd part is left blank as well as Announcement name 1st part.

Announcement name 2nd part is set to Title in order to include the forum main topics categories in the url:
This generates the following url : www.avi-plus.com/repair-tips-forum/miscellaneous-software/

The Add categories to URL is set All categories in order to include sub-topics categories like this:

Then the category name 1st part is set to Title in order to include the topic name like this :

Category name 2nd part is left blank in my case.

In the Common tab, Use Site Name is set to Title, Handling to Joomsef handler

Important for the Kunena "Thanks" button to work, include pid in the custom non-sef variables.
If you don't include pid, the Thanks button click will always forward to the same post in the forum.

In the Joomsef main configiruation (in Basic mode), I've added .html in the file suffix so that
the URL will be terminated in .html like this
www.avi-plus.com/repair-tips-forum/misce...rfacing/page-14.html

(page-xx.html when the topic continues over several pages.)

During the Kunena setup extension, always purge the SEF-url after each change in order to be sure to generate new URL.
I've also disabled : The Apache APC cache, the Joomla cache and the SEF cache.
When the settings are correct, the cache can be re-enabled.

So this works again and the URL's are generated as they were before.

Thanks to dajo for the additional information on these settings.

This is exactly what I'm doing, but to no avail up to now.
This week-end I'll try to reinstall Joomsef and the Kunena extension.

Hello,
Thank you very much for that additional information.
Unfortunately, and even with each cache level disabled, I don't see any change in the URL when I change many settings.
In fact most of them are not doing anything although I can see in the MySql DB that the settings are changing.
I'm going to remove completely Joomsef and reinstall the last version together with my license number and with the extensions.
I hope it will fix that strange issue and I will give some feedback soon.

I really don't understand how the switches are working.
It would be nice to have a schematic showing the various parts of a URL corresponding to each switch.
When I set the URL's like this :


It produces url's like this



It repeats twice the Category name 1st part Custom text !?
Am I doing something wrong ?

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)

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.

Here is the screenshot. Thanks for your help.

@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

Hello,
I've a similar problem.
I've just updated the Joomsef version and my URL have changed, although I've not changed any settings:
Example in my forum:
Before the URL's were like this (page 11 before upgrading):
www.avi-plus.com/repair-tips-forum/misce...rfacing/page-11.html
Since yesterday, after having upgraded Joomsef (page 12):
www.avi-plus.com/categories/forum/forum/st40-stb71xx-jtag-interfacing/page-12.html

Just to be sure, I've reloaded the Joomsef settings before the upgrade (sql backup).
Nothing to do, the url generation have changed.
Could help me to recover the same url's as in page 11 i.e. changing the /categories/forum/forum as in page 11.
Thanks in advance.

Quick, simple and efficient ! This was the good way to to that !
I've simply added pid into the Custom non-SEF variables box and erased the hundreds of duplicate "thank you" url's via the SEF url management.
Now the pid is included in the url and the "thank you" goes to correct user.
Thanks so much for your help.
Yves

BTW Could you quickly change my visible username=email to YLG80.
Actually everybody can see my email address, even if the option email invisible is selected.
When I get to the profile edition, the username is grayed out so it's impossible to change it.

config: Joomla 1.5.25 - Kunena 1.7.1 - Joomsef 3.9.3 registered - Kunena JoomSEF extension 2.1.3

Hello
I am using the config shown above for a Kunena forum
Each time when I click on "thank you" button (in any post) it redirect me to another post (usually each time it redirect to that same post) and thank give to that post where it was redirected instead of the post where i click the thank you button.

When I inspect the url's they are all the same in the same category like this:

../thankyou.html?91efb697ada5f8bd84dbe1e3a6c9c7b1=1
../thankyou.html?91efb697ada5f8bd84dbe1e3a6c9c7b1=1

If I go on the Kunena.org forum, there is a pid prepended to the url trailer like this :
../thankyou?pid=115144&72131becf8aff2075e4d45a3a900b374=1
../thankyou?pid=115152&72131becf8aff2075e4d45a3a900b374=1

Any idea to fix that issue with Joomsef enable woul be welcome?
Thank you
Yves