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It looks like the authcache code intends to not cache 404 errors, and the logic for this is based on the boost module:
However, in authcache.helpers.inc there was an attempt to "flip the logic around" in a way that broke it. The attached patch fixes it so only html 200 response codes are cached (unless explicitly listed to cache) the same as the boost module does it in the above commit. (Note that in the latest version of the boost module the logic in this area has changed considerably since the above linked commit)