When migrating an archive mailbox that is larger than 100GB with On Demand Migration (ODM), the following error may occur:
Could not save: "An internal server error occurred. The operation failed."
The Office 365 auto expanding mailbox functionality does not occur in real time. The ODM migration will start experiencing errors once the archive mailbox is full. The archive mailbox will need to increase in size before ODM will be able to migrate more data.
The following article from Microsoft includes more details about the auto expanding archive functionality, and some PowerShell commands to help trigger an expansion:
https://blogs.technet.microsoft.com/exchange/2018/04/09/office-365-auto-expanding-archives-faq/
Here is a snippet from the article:
Will the archive immediately expand when it reaches the threshold?
Expansion does not occur in real-time, in other words we only check if a mailbox should be expanded when managed folder assistant runs on the main archive. Managed folder assistant is expected to process the mailbox successfully within 7 days (it might be sooner depending on service load) or – you can initiate managed folder assistant on the main archive manually.
Currently, each time the archive mailbox is expanded, the ODM archive mailbox migration will need to be run again to fill it up, repeatedly until the migration is completed.
An Enhancement Request, ODM-762, has been logged by the product team to investigate ways to better handle this situation in a future version of On Demand Migration.
