modifying forum permissions

Hi,
When trying to modify the forum permissions in the admin control panel, so that certin user groups can post topics and others cant within specific forums i get the following error:

An error was encountered
Error: Unable to insert group forum permissions.

Currently, it is set up so that all user groups can read, no user groups can post replies and only mods can post topics. I can change the who can post topics with no error but I cannot change the post replies, that is when I get the error. This has just started and I don't know why, since it working and not working I have not modified any code, just been adding new forums.

As a secondary problem following on from this, bearing in mind the permissions above, all users can post replies and post topics when only mods should be able to post topics,,,, very confusing

thanks for any help anyone can give