We get error when we try to move mailboxes from exchange 2007 to exchange 2010.
These mailboxes have been started with new-moverequest with -SuspendWhenReadyToComplete option. When we resume moverequest to migrate the last 5% of the mailbox we get at failed mailboxmove with an errormessage: ”Unable to delete folder”.

You can try to remove your current moverequest and make a new moverequest. This time move your mailbox in one hole pieace without -SuspendWhenReadyToComplete option.
This works for me!