Hi @mrtro-1199 ,
As it works with OWA, does this not point to the Outlook client itself?
Considering it could work normally on OWA, the issue may be more related to your Outlook client, what's the version of your Outlook 2016(File>Office Account>About Outlook), please first ensure that you have updated to the lates version of Outlook.
Was there an error prompt when you delete the item? Considering the issue only occurred on the specific calendar items, are there any similarities on these items(type, Organizer, meeting time or others)?
If there would be an error prompt that displays you don't have permission to perform it, it's suggested to check your calendar permission via right-click your calendar>Properties>Permission or re-assign the permission to test.
Besides, in order to avoid the interference of add-ins, please try to restart your Outlook in safe mode to test(Windows + R>type "Outlook.exe / safe">Enter).
If the issue still exists, in order to further confirm it, it's also suggested to assign the permission to other user or configure the user B's account to other client to check if the issue has any difference.
If your issue has any update, please feel free to post back.
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