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Hi Zion,
Thanks for your updates.
In this post, I need to focus on the issue in the calendar list. If you are experiencing other types of issues, I suggest you post a new thread on our forum for expert help.
I’d like to confirm whether the issue happens in a specific calendar list in the site. If you do a test in another site, what’s the result? I also suggest you do a test in a fresh browser to check the result.
Meanwhile, as the original issue is that you can’t delete the list item, I’d like to confirm whether you can delete the affected item and create a new one.
Thanks for your understanding,
Cliff