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. 

oncehub check ews