Here are a few tips on troubleshooting issues with db4o.
To get more diagnostics information you can register diagnostics listeners.
Take a look at this small guide which gives tips on how to deal with a corrupt database. See "Deal With a Corrupt Database"
If you experience issues with the performance of db4o take a look at this topic: See "Performance and Scalability Issues"
If you hit a problem you can ask for help in the db4o forums. You can find the forums here: http://community.versant.com/Forums.aspx