One of my admins cannot see his PM inbox. I logged in as him and am having the same problem however my inbox works just fine so it has to be something with his account. When I click view source there is no html at all and the page simply renders blank.

Do I need to purge his inbox? If so, how can I do this?

Can you verify what version you are running?

6.5.5 but I just upgraded to the latest and it seems to work now