I have a myBB forum which is returning errors when I alter the permissions of any of the forums or categories. On Chrome this manifests as a 500 server error and on Firefox it just gives a blank white page.

When I alter permissions it appears to change the permissions but the alteration is sometimes blind, ie it is shown as changed but nothing actually changes. When I delete forums it refreshes to show it has managed to delete the forum.

I've checked the server logs and the only error our host could find relates to a different script for updating Steam and XBL online information (which is now disabled as I fix the permission bug first). I have also tried deleting tons of forums and groups incase it was an excessively long mySQL operation causing the problem; and also optimizing tables and rebuilding the cache. I did notice that deleting forums, groups and optimizing did not decrease the size of my forumpermissions cache (from 4.85mb)

We have no errors in the day to day usage of the site, or any other part. Only where group permissions are involved do we have errors.

I am woefully underskilled for dealing with this type of problem and the myBB support are slow, unhelpful and jaded by quantity of aid they have to provide. Therefore I thought I'd take a shot here.

Thanks for your time taken to read the above,