Others cant see a period of users calender

We have a user "XX" - if i open his calendar in my outlook (both web and app) i can see his entries, but not in a certain period - so before january 10 mi can see his entries and after march 27 i can see them. But all entries between these two dates are not visible to me (or to the other 2 random people i have asked to open his calendar.

Do any one have some idea what could be wrong/what i have to check?