There is another interesting question about status 303 and seo here but it does not answer me why status 303 appears now in GWT in the crawl errors section since end of January. In my understanding 303 status is NOT an error and there are situations where 303 is from the technical point of view the right thing to do.
See explanation of the status 303 (See other location) here:
The server returns this code when the requestor should make a separate GET request to a different location to retrieve the response. For all requests other than a HEAD request, the server automatically forwards to the other location.
So is the final conclusion that 303 is evil for SEO? Should only 301 redirect be used?