Drupal 7, Aggregator, RSS no longer imported: The feed from $ seems to be broken, due to an error "Invalid document end" on line #
It turns out that a simple smiley :) wrought all the havoc. The smiley was situated in the first paragraph of a post and was therefore transferred via the feed to Aggregator.

Removing the smiley removed the errors and now the feed is showing correctly again.

I don't know what exactly is wrong with ':)', but somehow it messed things up. Explanations are welcome.