As of now, the issue has been solved by Joomla support.
It seems however, to be – something which has nothing to do with this site in itself, but a rather tecnical thing which they call a ‘bug’ – meaning a tecnical thing in the programming which is way beyond my understanding.
In fact, it seemed, there was a dot too much in some programming line 🙂 – which is either Joomla or Kunena Forum’s responsibility. These things seems to happen in the it-world.
Unfortunately the issues has been solved for now, – but will return when Joomla updates next time. So in the meantime I will still have to figure out how to avoid it happening again.