I have the problem again.
I tried to load other profile, no way to make it work.
When i change the search engine, normaly it has to write the configuration somewhere, but it does nt

Given that only you seem to be striking this issue - could you try reproducing under a new user?
If you still can reproduce there, can you test it under a Live CD of another distribution such as OpenSUSE or Kubuntu to ensure this is not a distribution packaging defect?

I created a new user. In the beggining everything works fine. I choose a search engine, i close konqueror, i reopen it and he remember it.
After i started to add some custom search engines, i surf on the web, i try the first the second, i colse konqueror i reopen it , ok everything fine.
After i change the setting "Use preffered engines only" i checked it , i unchecked it and at one moment i reproduce the bug, i have no search engine after relaunch konqueror and even if i choose one, i type sometrhing in the search bar, i hit enter nothing happens.
I have the same symptome some times in my main user, i type something inside i hit enter and nothing happens.

I think that to reproduce it you have to use the search bar and change engines, settings etc

Until now i did nt find excactly the steps to reproduce it.

It seems that the setting "Use preferred engines only" has some important effect here.
Under a new user - without changing that setting at all, does it work normally?

I had noone search engine in the search bar, and to make it work again, when i was in file explorer mode i choosed a search engine, and i close konqueror in file explorer mode. After relaunch konqueror ( i launch konqueror always with the web borwser profile), the search bar works fine (until next time that will loose its settings...)
So, i will try to verify it again, it saved the setting only when i was in file explorer mode. When i am in a web site and close konqueror, he did nt save the last choosen search engine.

I see, that behaviour is quite confusing - it seems the search bar part of Konqueror may not be respecting the profiles feature.
If you could verify the behaviour, that would be great.

Did you see something strange in your installation too ? Because i would need some help to determine what happens here

Unfortunately I could not reproduce the issue you reported on my system - Konqueror worked fine.