@Yini Zhang,
Given that this started after the migration of your accounts to Exchange Online, I'd suggest checking the status of the
Use Cached Exchange Mode and
Download shared folders options for each account again. I know you said they're enabled, but just verify that they are enabled for your business account and not just the bot account. That's the only thing I can think of that would cause issues with getting access to folders.
If they are enabled try disabling and restart Outlook. Then re-enable them and try again. If that doesn't solve the issue, you may need to consider looking at the Microsoft 365 connector for Outlook. It uses the Microsoft Graph API to work with Exchange online instead of trying to work through the local Outlook instance.
Cheers,
Eric