Well this is a classic forum and it looks like you have threads, Don't know what version you have.
But anyway the error usually means you have a typo in the named script of the error message.
Could be the file is corrupted or somebody modified it incorrectly.
So that is why Gizmo suggested to download the software from the members area here and replace that one file:
viewmessage.inc.php
With a virgin untouched copy.
