I'm using the latest version of both WordPress and Akismet. Someone pointed out to me that the comment form on posts wasn't working. After troubleshooting I discovered, that it does work, but only if you are already logged into WordPress. If you are not logged in, the page just reloads after submitting a comment.
If I deactivate all my plugins it works for visitors. But the moment I activate Akismet, the comment form quits working. This is with no other plugins active. I regenerated a new key, but that didn't solve the issue either.