8
Can you go into Administration Menu>System>Preferences>General Settings and set "Debug Mode" to "Enable" (inline or popup, you chose).
Report back any errors.
Also, try using the default theme and see if you get the same problem.
Edit:
On my test site (php5.2.5, cbb 3.08, Frameworks 1.22, xoopseditors 1.21) I've just successfully made a new category, put a new Forum in it and posted a test message in the new Forum. You problem isn't down to CBB.