Fix #14122: Revised message for error 2800

The message now reflects the fact that an invalid security token error
is often triggered because of a session timeout.

lang/strings_english.txt

@@ -294,7 +294,7 @@ $MANTIS_ERROR[ERROR_COLUMNS_INVALID] = 'Field "%1$s" contains invalid field "%2$
 $MANTIS_ERROR[ERROR_SESSION_HANDLER_INVALID] = 'Invalid session handler.';
 $MANTIS_ERROR[ERROR_SESSION_VAR_NOT_FOUND] = 'Session variable "%1$s" not found.';
 $MANTIS_ERROR[ERROR_SESSION_NOT_VALID] = 'Your session has become invalidated.';
-$MANTIS_ERROR[ERROR_FORM_TOKEN_INVALID] = 'Invalid form security token. Did you submit the form twice by accident?';
+$MANTIS_ERROR[ERROR_FORM_TOKEN_INVALID] = 'Invalid form security token. This could be caused by a session timeout, or accidentally submitting the form twice.';
 $MANTIS_ERROR[ERROR_INVALID_REQUEST_METHOD] = 'This page cannot be accessed using this method.';
 $MANTIS_ERROR[ERROR_INVALID_SORT_FIELD] = 'Invalid sort field.';
 $MANTIS_ERROR[ERROR_INVALID_DATE_FORMAT] = 'Invalid date format.';