I have the same problem with the same permalink structure as aarnet above, /index.php/%postname%/. Using the default permalinkstructure (?p=123) I can view pages, but when I use the structure (/index.php/%postname%/) I’ve used this last year, pages gives a 404. It works just fine for posts though.
I’ve tried deactivating all plugins, switching themes, and updating the permalink structure, but the problem is still there.