Outlook Calendar integration not working

Hello

Our Outlook integration appears to be getting stuck during processing. This was working before but now it is not sending out calendar invites. When employee opens the enrollment, message appears "we will add this event to your calendar soon. First, we need to process it."

I have verified that the Sync Instructor-Led Training for Learning is running each hour and it is showing successful.

Does anyone have suggestion ?

Thank you in advance

Jennifer