I don't understand why, but my Drupal messages won't disappear, they just get "added" to each other. Someone has posted a similar problem on drupal.org: "warning/error messages won't disappear", but no solution has been posted yet. There's another one which is almost the same: Drupal messages won't go away.
Here's an example of the problem:
I do have the Administration menu installed, but the problem didn't occur earlier, so I don't think this is the source of the problem.
I've already cleared cache, ran update.php, ran cron, so I'm out of ideas.
I recently installed Ubercart module, but I don't know if it has anything to do with my problem.
But there's another one which I also don't understand: if I try to open a page which doesn't exist, I don't just get a "Page not found" error, but many MySQL errors too, which looks like this: (1) (2). It's really unhealthy too... Maybe there's a connection between these problems.