Insufficient permissions

Now I know the issue.

Active users are kept in an application variable if the application pool for your website is recycling then all active users will be dropped and you will also get the Insufficient permissions problem that you mention.

First you should check that your application pool has a single worker process, then check the following to prevent the application pool recycling:-

1. Disable rapid fail protection
2. Check that private and virtual memory limits are not set for recycling the application pool
3. Make sure that the application pool is not set to recycle after number of minutes and is just set to your 00:15 time
4. Make sure that another application running on your website is not using session abandon, which will cause all sessions to be dropped (chat rooms often have this, but the one built in to web wiz forums does not)

You may want to place the forum in it's own application folder and give it it's own application pool to ensure that other applications on your website are not causing problems with it.

If you still find you have problems with the app pool recycling after all this you should get on to Microsoft support as there is probably something wrong with the server.