Errors migrating a public folder: "Too many concurrent connections opened., Cannot open mailbox" (4331985)

Issues migrating a public folder. We have two migration events running in addition to this migration. What is causing this?

Too many concurrent connections opened., Cannot open mailbox. server = <value>.<value>.prod.outlook.com, user = /o=ExchangeLabs/ou=Exchange Administrative Group <value>,
 mailbox Guid = <value>
ErrorCode = ErrorTooManyObjectsOpened

Follow these steps to increase the throttling for source and target tenants:

• Logon to the Office 365 portal
• Click Admin icon to open the admin center
• Click on the ? on the top right
• Click in the text field and search “EWS throttling”
• Click the Run Tests button
• If EWS is being throttled a prompt will be displayed to select the number of days to relax throttling.  The max setting is 90 days per request.
• Click the acknowledgement checkbox
• Click the Update Settings button

When the settings are updated the following message will be displayed.
Success!
An update to your Exchange Web Service (EWS) throttling policy has been initiated. The update typically completes within 15 minutes.

If this does not work, submit a request to Microsoft for EWS throttling policy for both the source and target tenants.

https://support.quest.com/kb/265794/request-an-upgraded-office-365-throttling-policy-from-microsoft-to-achieve-best-migration-throughput-and-performance-265794