Hi @GlenTech ,
Welcome to our forum!
According to your description, i notice that after you rebuild the .ost file, the issue disappeared briefly. And i notice that the issue is related to a user, in order to further confirm if the issue is related to the current Outlook, it's suggested that you could add the user's account to other computer without problem to check if there're any differences.
If the everything i ok on other computer, considering the .ost keeps being corrupted, it's suggested that you could create a new Outlook profile(Control Panel>Mail>Show Profiles>add) to check if there're any differences.
On the other hand, if the .ost file is too large, it may also affect the normal operation of Outlook, please check if the size of the .ost file is large.
If the issue persists, considering the impact of some third-party programs on your computer, please try to perform a clean boot for windows and check if the issue still occurs.
If the above doesn't help, considering that it would also be affected when the outlook program itself is damaged, it is recommended that you try to fix the Office application.
Hope to help you!
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