First-level permalinks not working

I have a problem with all “first-level” permalink. A subpage with permalink “first-page/second-page” works, while “first-page” alone returns a 404. Same problems with posts if I use “/%postname%/” as permalink structure.

Moreover, while testing I discovered that if I use as permalink structure like “test/%postname%” (or any other fixed term) all posts work with the structure test/post-slug and, unbelievable for me, first level pages started working.

Has anybody encountered a similar problem?