Some of the mirrors out there might have stale files. This happened to me recently, and it was in also tied to the caching server I'm using (apt-cacher-ng) which tries to save bandwidth by redirecting the repositories for same archive to a single entity (in my case if was a Hungarian mirror). Direct updates through German mirror worked ok, for example. Try changing the mirror you're using. In case you're using apt-cacher-ng, you'll need to do something in the line of changing the following file's contents:
After that you should also restart apt-cacher-ng for changes to take effect.