Hi jLiangCMHA,
Based on my experience, you can try to turn off the cache mode and check whether it works, it will update the mailbox with the server that runs Microsoft Exchange, for avoiding Outlook running slow, remember to re-turn on the cache mode after the process.
1. Click File > Account Settings > Account Settings.
2. Click your Office 365 account, and then click Change.
3. Under Offline Settings, uncheck Use Cached Exchange Mode.
4. Exit, and then restart Outlook.
The further detailed steps you can also refer to this link:
Moreover, to better help you, I need to collect more information for further investigation.
1. Please try to open the outlook in safe mode which disables the add-ins and check whether user A can access this folder successfully.
2. Is the user A was configured as a primary account in Outlook client?
3. What’s your outlook client version?
Regards,
Bryan