UBB.threads PHP Forum Software Community Forums UBB.threads 6 Version 6 Support Error Message "We Cannot Proceed."

This may be a common question with a simple answer. In one of our Forums, no one can post. The following message appears:
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We cannot proceed.

The form you have submitted is no longer valid.

Please use your back button to return to the previous page.

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The correct permissions are given in UBB Threads' Control Panel, so I cannot determine the cause of this problem. Can you help asap? The users of the Forum have been waiting patiently.

Thanks!

Tracy Schlee
PsyBC