Is it a first install (no upgrade from 0.5)?
Have you used uddeIM before? Have you installed other tools that write into uddeIMs database tables?
The first message is shown when a user is not logged in and tries to access a message, the second when the message has already been deleted (but then it is also not shown in the outbox).
Have you enabled caching in Joomla? Maybe there is something messed up so an old page is displayed?
uddeIM & uddePF Development
CB Language Workgroup
CB 3rd Party Developer