Outlook Classic + iCloud (Store App): "Malicious Code" error on Appointment Creation. MFCMAPI confirms standard IPM.Appointment class.

After extensive testing and confirming that neither 'Trust Center' settings nor 'Whitelisting' scripts solved the issue, I found the root cause:

The iCloud Form Region (the custom UI layer Apple injects into the Appointment window) contains code/ActiveX controls that trigger modern Outlook security blocks.

The Solution (Workaround): Instead of fighting the security block, you can simply disable the visual Form Region in the Registry. This forces Outlook to use its native/standard Appointment window (which loads instantly and safely) while keeping the iCloud Synchronization engine fully active in the background.

Steps to fix:

Close Outlook.

Open Registry Editor (regedit).

Navigate to the Form Regions key. Depending on your Windows/Office version (32-bit vs 64-bit), check both locations:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Office\Outlook\FormRegions\IPM.Appointment

HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Microsoft\Office\Outlook\FormRegions\IPM.Appointment

Look for the key/value named Apple.DAV.Appointment.

Rename it to _Apple.DAV.Appointment (adding the underscore hides it from Outlook).

Restart Outlook.

Result:

No more 'Malicious Code' error when opening/creating iCloud appointments.

The window opens instantly (Native Outlook UI).

Synchronization to iPhone/iPad works perfectly.

Note: A future update of the iCloud for Windows app might recreate these keys. If the error returns, simply rename them again.