We have a site set up in SharePoint Online that inherited a couple of classic calendars from its on-prem original site. We've created a new site in SharePoint Online and used ShareGate to migrate the contents from SharePoint Online A to SharePoint Online B. The calendars were migrated and I can see in the site contents of the new site that all the items came over. The problem is that if anyone tries to open one of the old style calendars in SharePoint Online B they get this message:
Any advice on how to get past this message so people can access the calendar?