Hi @JoFri ,
Are you using Outlook desktop client? If yes, what's the version of your Outlook? (File > Office Account > About Outlook). Please make sure you have upgraded your Outlook to the latest version.
And does this issue only happen to your specific Outlook desktop client? Do other user in your organizations have the same issue?
I tried some research and found some possible factors, please try the suggestions mentioned in this article Some rules are disabled, and you receive an error message when you try to create or enable rules in Outlook and see if your issue has any difference.
Any update, please feel free to post back.
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