Outlook calendar does not sync

Hello,

We are using a shared Outlook calendar that belongs to one person, and the others can access it and create, move, or delete appointments there. One colleague somehow (I have no idea how) deleted all the appointments in the calendar, except for 3–4 new ones that she entered at that time. These appointments appeared in her "Deleted Items," and she has now restored them all. She can now see all the appointments again online in the shared calendar (sometimes even duplicated), but the others still cannot see the deleted appointments.

When she tries to change or re-save an appointment, however, she gets an error message.

What can I do? There were many appointments scheduled for next year, and I don’t want to recreate everything from scratch.

Thanks,
Best regards,
Martin