I am having EXACTLY the same problem as one that was posted 11 months ago (and is also reported on numerous sites on the Internet), however, a volunteer moderator just closed the thread that I posted to and IGNORANTLY stated that I don't have the EXACT SAME problem (no, I'm NOT happy about that, especially after I've already WASTED many hours on this). In addition, he closed the thread, even though I'm having EXACTLY the same problem. So here it is, AGAIN:
"All pages on my site, aside from the homepage, are giving me 404 errors. The pages definitely exist and the permalink structure is correct."
The only way my blog works is to set Permalinks to Default but as I've now had the blog up and running for quite some time, I don't want to do that.
The following "solutions" DO NOT WORK:
"Can you try once saving permalink structure again to default. Check if it works:
Admin > Settings > Permalinks
If it works, swith it to desired permalink option and check it again."
Changing permissions of file directories DO NOT WORK.
Changing the .htaccess DOES NOT WORK.
Again, this is EXACTLY the SAME PROBLEM as one reported 11 months ago and many others are reporting it elsewhere.