My first guess would be that this user is somehow not linked to a default queue. The link is via the queueid attribute on the systemuser entity. I don't think you can query on this attribute in Advanced Find, but the following SQL query should identify this:
Select * from filteredsystemuser where queueid is null
As to how it happened, that's difficult to know - was there anything unusual about how the user was created (e.g. created via the CRM API, rather than the CRM UI) ? Similarly
And as to how to fix it. If a suitable queue exists, then it should be possible to set the queueid on the systemuser record. There are 2 ways to do this:
1. The supported way would be to use the CRM API to do an Update request to set this
2. The unsupported way would be a SQL Update statement
If there isn't a suitable queue, I think the safest option is to create a new user, though this would need a new login name. Otherwise you might be able to create a new queue, or link the user to another queue, but either of these may cause future problems
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