Office 365 (Known Issues) - Importing ICS Files in Outlook
There is a known issue with importing ics files within the Outlook Web App and Outlook desktop clients for both Mac and Windows. Message displayed: The events couldn't be imported. Please try again later. Error message: An internal server error occurred. The operation failed.
Open the .ICS file in a text editor like Notepad for Windows or TextEdit for Mac, then search for METHOD: REQUEST and change that to METHOD:PUBLISH, save the changes and proceed to import the file as you were doing it.