Cause:
A Disabled Account does not have rights to its own mailbox.

Fix:
Locate the disabled user in AD users and computers > Right click > Properties >
Exchange Advanced > Mailbox Rights > Locate the SELF object > "Associated External
account" > Allow.

UPDATE: If you have lots of these you can repair them with the NOMAS tool.

Multiple errors

Use NoMAS

Comments:
This is more of an annoyance than a problem - but if there are a lot of these errors
it can kill the server.