Outlook 2000, msg. Outlook cannot find seriall information "XX", won't diplay new app.

[Darkmoon]

Outlook 2000, when clicking on the calendar, the message
Outlook cannot find seriall information "XX" is diplayed. All old
appointments are displayed but not "XX"

When entering new dates, nothing is displayed. But when exporting either to
a pst file or a excel file only the newly entered and not displayed
appointments are shown.

It is not possible to Edit the old data.
scanpst does not change anything, file size arounf 260 MB, so tthat should
not be the problem.
As this is a Gemran outlook 2000 on a Gemran W2000 computer I translated the
above message and it might differ a littel on an English System.

Any idea what to do?

Kind regards
Judy Ann