To work around this issue,
#1 close all items after you run the code to add a UserProperty object to an email message.
#2 Or, run the code only one time for the same email message.
This issue occurs because editing and saving an email message multiple times for an IMAP account is inefficient. When an email message is saved, it must be uploaded to the server as a completely new email message, and the original email message is deleted. If the preview pane is enabled, the original email message remains in memory, whereas the underlying email message is changed because of the multiple uploads.
https://learn.microsoft.com/en-us/previous-versions/troubleshoot/outlook/0x80040109-outlook-object-model-used-with-imap-account
Ich würde als erstes versuchen die Vorschau-Ansicht vor dem Ausführen des Makros zu deaktivieren.