Postings.php Thread Error

A few mods on my site are getting this error when they try to close threads and open threads etc...:

PHP Code:

The action you have attempted could not be performed as your session appears to be invalid. Click the below link to attempt this action again with a new session. Try this action again! 

any ideas whats up??? this week ive installed the modlog hack and the sticky on new thread hacks....

- Mist
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Also if you have time can someone check this out for me too

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[g-force2k2]

could be something with the server... cause i know i used to get them kind of errors... its just like invalid sessions or something... regards...

g-force2k2

[Dean C]

weird....

- Mist