Do you mean manually on my computer, or by logging out? :)

I mean clear them in your browser.

Tried it and it worked thanks! :)
And one question: why do some thumbnails appear in a quote under the message "attachments", and other thumbnails appear just within the thread?

Nice troubleshooting Lynne.

Because some users attached the image inline and others just left them as regular attachments.

This exact issue came up just a couple of weeks ago here, so I immediately recognized what was happening when I was able to view the page source. :)

I have the same error. I tried all solutions above but I still cannot see attachments. Here's an example:
http://www.wp7.nl/forum/f17/algemeen...html#post24085

In my case, thumbnails for attachments have been working fine since I set up the forum back in April. But suddenly, thumbnails don't show unless they are of the png file type. JPG and JPEG used to show fine, but now when you upload a JPG all you get is a link. If you upload the same picture as a PNG it shows fine. And nothing has changed to the configuration of attachments, so I'm befuddled.

it could be an update vb made if you recently updated anything...

you need to check attachment configuration from admin cp and be sure you have the right configuation, otherwise, nothing will work.

also make sure about the path where your attachment is stored.

and last, make sure about usergroup permissions.

Good luck