Using older versions of Outlook for Mac for personal email and calendar
Our team is also experiencing this issue currently.
Some of us are running OSX 10.13, the solution I am rolling out is updating to Mojave (last version supporting 32bit and 64bit apps)
Once this update is performed we are updating/reinstalling all Office 365 apps to the latest version, it appears that the issue is not only limited to Outlook and in-fact Word and Excel are effected also.
Strangely it did not effect all users at once.