As Steve said above we have had a small number of users for whom upgrading from a V4 to V5 system has scrambled their permissions – and so far have been unable to trace the issue. Not helped, of course, by the fact that it is random and only happens to a few. We are sorry about this and would love to be able to fix it!
Now – the solution depends a little on the complexity of your data. If you have a fairly straightforward forum setup without complex combinations of user groups and permission sets then I would suggest you go to the forum admin > Permissions and use the Reset Permissions tool. But this does mean you will then have to visit the Manage Forums panel and re-apply the User Group/Permission relationships. Not usually a difficult or long task for a straightforward set-up. Just annoying I guess. But it is the simplest and quickest if possible.
As always – I would suggest making a backup before you go for it though. And if you feel your set-up is too complex for this come back and we can rethink.
