I never used ver6. When I visit your site I also see the hop to lists is empty.
Digging a little more using active topics I see the forums but thats it. Only the current forum the post is in.
You said you fixed it the last time with a backup.
Are you talking about the forum files or the MySql tables or both.
Maybe the mod you think you have is the problem?
I know I had a similar problem with ver7x due to what is called safemode in php. Once it was disabled the problem went away.
My problem was the styles, everytime I edited them it corrupted the table.
If the forum files are getting corrupted.Probably a host issue going over quota on disk space?
Maybe it is time to consider upgrading to ver 7.5. It will be out in a week or two. And it has sub forums built in.
