If you're performing a Staged or Cutover migration to Exchange Online you may run into the following error:
Error: MigrationPermanentException: An error occurred while running Get-MergeRequest -Identity : Delete() is not supported on a read-only session. --> Delete() is not supported on a read-only session.
Many customers reported this issue in the last couple of days. Moderators in the Office 365 Support Forums confirmed that Microsoft is aware of the issue in their backend and is still investigating. Unfortunately the Service Health dashboard does not make mention of this issue.
If you run into this issue you could try to stop and delete the migration batch, delete the created Office 365 users and restart the process. Some people reported their migration to succeed now. Others are still seeing the same issue in their migration batch.
To make sure that Microsoft has a good understanding of the scale of the issue, please open a Service Request if you're impacted too. And keep an eye on the discussion thread in the support forums to see if there's any progress made in resolving this issue.
Update July 14th 2015
Microsoft confirmed they implemented a fix in their environment, but it may take some time before it applies to all tenants. To find your tenant version, connect to Exchange Online with PowerShell first. Then query the version number:
Get-OrganizationConfig | ft AdminDisplayVersion
The fix has been implemented in version 15.1.234, so if your tenant is on that version or newer you can restart the migration batch and expect it to no longer fail. If your tenant is still on an older version I'm afraid you just have to wait a little more.