I've got a particularly strange problem. My Custom Permalink is: /%year%/%monthnum%/%postname%/ and it functions perfectly.
When I change the permalinks to anything else i.e. /%postname%/ the links show as they should for example http://www.mysite.com/postname/ but I get a 404 error. I change it back to /%year%/%monthnum%/%postname%/ and it works again.
The only other Permalink configuration that woks is the "Default" i.e ugly URL.
It doesn't matter what Theme I use or how many times I update the .htaccess I get the same 404 error. I even disabled all the plug-ins and the problem persists.
I have installed a clean copy of WordPress with the theme and was able to change the permalinks at will so there is something wrong with my current set up.
Does anyone have any ideas?