If you have your once scheduleonce connection with Office365 disconnecting and having issues authenticating the connection. It means that you are using the old EWS connection. Recently Microsoft has discontinued this. I would advise you to Disconnect the current connection and reconnect Office 365 using OAuth.
To do this:
Log into your OneHub account >> click on the initials/ profile picture in top right-hand corner >> Calendar connection >> click on Disconnect >> from the same page click on Connect for Office 365 via OAuth 2.0 >> enter their credentials to connect the calendar.
To check if you are currently using EWS, simply hover over the office 365 connection you currently have from within ScheduleOnce.