KMail doesn't see imported S/MIME certificate, certificates (by postsignum - authority in Czech republic) are OK (succesfully imported and used in KMail on another PC).
Workaround, I had to remove settings files for kmail etc.
KMail still doesn't see certificates S/MIME made by czech signing authority PostSignum.
It is crucial to import all parts of certificate (root certificates, paired key etc.)