CI 3.1.2 Unable to load any controller but default

It should work on 5.3.7 as well, but we strongly advise you NOT to run such old versions of PHP, because of potential security and performance issues, as well as missing features.

Yes, you are right, I'm just testing CI 3.0 to see how it works on a private server though, when the time comes to start developing, I'll use a much newer environment. Thanks for the hint though :-)