Hi krwelliver,
From the screenshot, I notice other users’ calendar cannot be opened. To find the root cause, I’d like to confirm some information:
1. You mentioned “set all sharing permissions in the MS Admin Dashboard to be allowed”. What kind of permission have you set?
2. Can you open shared calendar Outlook Web App (OWA)? I suggest you sig in to your OWA, and then try to open other users’ calendar. This can help confirm if this issue is related to Outlook client.
Thanks,
Young Yang