I just started looking at Grocerycrud. I downloaded CodeIgniter 3 and installed GroceryCrud and looked at "The newbies grocery CRUD Tutorial" and followed the instructions there and got a 404 error when trying to call http://localhost/Cod.../index.php/main (the main.php controller in the example on that page). I then made sure that the file permissions, ownership and the group the page belonged to were the same as the other CodeIgniter controllers and tried again, but still got a 404. I then downloaded CodeIgniter 2 and called http://localhost/Cod.../index.php/main and it worked immediately out-of-the-box. I downloaded CodeIgniter 3 a second time just to make sure and still got a 404. I've been using CodeIgniter 2 at work for about a year and a half and I'm pretty comfortable with it.
A fresh Lubuntu 15.10 with a standard LAMP install (on VirtualBox).
I get the Apache welcome page and the standard CodeIgniter welcome pages for both CodeIgniter 2 & 3. I installed Firefly and watched the console output and it was just a "not found" error.
Thanks in advance for any input!