invalid attachment problem

[Macsee]

When I click on the image in posts, I get the error message
"invalid Attachment specified. If you followed a valid link, please notify the administrator"

This is something I've noticed over the last few weeks.

Strange thing is that someone created a post with an attachment a few days ago and it worked the first time I tried it. But when I go back to that post now I get the error message.

This seems to affect only the private forum (Admin CP > Forum Manager > Show Private Forum = Yes, Display Post Count).

I've tried a site: search in Google to see if the problem has come up before on these forums, and I've found a few threads about the issue, but no solution.

Any suggestions?