Calendar-Access via DominoWebAcess not possible

Hi,I´ve the following problem with some users, who gave access to other users to read their calendar and the ToDo via the internet.

The user can connect to the mailbox with their internet password. They click on the tab for “ToDo” - so the tab opens and they can read the entries. But: if they click on the tab “Calendar” the log-in window repeats and the tab doesn´t open. When they try to log-on nothing happens. The log-in window repeats again. This problem appears only via the internet, opening the calendar via lotus client is no problem.

Can somebody tell me where the fault?!

Thanks in advance !!

Every tip is welcome , Stefanie