500 when trying to save settings

[choccyness]

I have a website that works fine on localhost, and works fine on my old host.

I'm now trying to move it to a new host like this (per the docs):
* Install a clean copy of GSCMS
* copy /data, /plugins /themes, gsconfig, and .htaccess from my localhost copy
* login, and set the website url.

But I'm failing at the last stage. I'm getting a 500 when I try and submit the new url (or in fact, try and save that page in general, even if nothing has changed).

I don't seem to be able to find a log anywhere which indicates what the actual error is.

The new host is HostPapa, accessed via cPanel and FTP.

GSCMS 3.3.13
Apache Version 2.4.27
PHP Version 5.6.31

I've confirmed all the files in the root of the directory are 755

Does anyone have any thoughts?