Bad advise to use Vasoline as that is a brand and doesn't fully cover what you need to use. You want to use petroleum jelly which is what Vasoline is but with the extra of flavor or scent which is NOT what we need. Petroleum jelly WITHOUT ADDED SCENT, etc is recommended by Eheim and works great as a very simple, cheap and easy item to keep on hand. Menthol is not a good thing for tanks.
Where many get confused is the name having petroleum in it, but if you thing about it a bit, plastic is also a petroleum product!
Rather than an impeller problem, I would have guessing it was a priming problem, so a question about how it was done might lead to the correct answer. Was the priming done per the manual or was it just kind of from memory, etc?
Sounds like the wrong routine was done on priming to clear air from around the impeller and impellers will not pump air but simply set there and rattle!
This is a game where small details, ignored, can lead to big trouble.