Office 365 Business Premium, Outlook: Selected calendars are reverting after switching between email & calendar or restarting Outlook

[MSCom]

We're using Office 365 Business Premium. Since last week all our Office 365 user are facing the same problem with Outlook. The previously selected calendars are reverting everytime the user switches from email to calender or restarts the outlook. Then only the user's calendar is selected and the users have to select the calenders again manually. Does anyone have the same issue?

We already tried to update the Office but it didn't help.