The answer posted in May of 2008 is correct, except that the image of the paper guide is missing from the posting. You can find the image on the Lexmark website. I'd like to add that the paper guide can easily get bent even when it's firmly in place. And this will also cause the error to appear.
If it's bent by even a few millimeters, it won't fall back into the correct position after a page is printed. So you can find yourself in a situation where the printer will print one page, the error will appear, you'll jiggle the paper guide and it will print another page, but then the error will appear again, repeat cycle..etc...
To fix this, very carefully bend the guide back into the right shape. You can do this even when it's already situated inside its holders. The lever portion of the guide should be parallel to the plastic frame above it, in either the up or down position. If the guide is even slightly bent so that one side is pointed a little downward or upward, that will be enough to trigger the error. Bend it so that it's parallel, and this should allow the guide to move normally again when a page comes out. You'll know that it's working correctly when it tilts up as a page comes out and then goes right back down afterwards. If it gets stuck in the up position, then it's probably still too bent or it's not correctly positioned inside its holder.
The guide is a fragile piece of plastic so bend it very carefully!
Colleen McGrew, March 2010