When I run the following code in Firefox it runs correctly, but in IE8 it says xpath cannot be evaluated or does not result in a WebElement.
webDriver.findEl
Could you try as below?
webDriver.findElement(By.xpath("//input[string(@id)='submitForm']")).click()
For IE doesn't have native XPath support, WebDriver use a third-party library called javascript-xpath for this, this may be a bug of it.