It seems I have solved this problem.
Before installing JomSocial I deleted the menu item "JomSocial toolbar" in the menu manager. After installing the JomSocial it worked.
I think that you better remove this menu item from the "quickstart".
Thanks again for your help.
P.S.
Where can I see information about module positions as in a demo?
Sorry for such simple questions. But I don't know much English and it's hard for me to look for information.
Thanks.