When I open a mail and try to reply, I get only a blank email (no quoting). Even if I select part of or the whole text, I only get a new mail without any quote. I would expect that the new mail includes the selected part of the old mail.
How can I do that?

Thanks for any advice

Edit your reply templates (Settings->Configure KMail...->E-Mail Editor->Standard Templates), there should be "%QUOTE" somewhere in there to quote the original text.
Since that should be there by default you could try to just click on the "Defaults" button and choose "Reset All Templates".

It worked. Thanks again for your help.
Seems that my template settings where all set to %BLANK.