I just ran into the same problem yesterday. I upgraded to 2.4.0 and it was running fine, then I built my own template and wanted to give it a test run. So, I uploaded the template and changed the _config in "mysite" and now the site's going through endless redirect loops.
I've dumped the cache, ran dev/build/?flush=all, tried opening the site on different browsers to make sure it wasn't a cache issue, and I've fiddled around with the _config file some more in hopes that I could get the site to stop redirecting. No luck so far. If it helps any, the following is the content of my _config file:
$project = 'mysite';
$database = '';
// enable nested URLs for this site (e.g. page/sub-page/)
// This line set's the current theme. More themes can be
// downloaded from http://www.silverstripe.org/themes/
"cunardclassic" is the name of the theme I'm trying out and have uploaded into the templates folder, so I'm pretty sure it's in the right place. I've also thought that maybe it was a template issue and tried redirecting _config to the default Blackcandy, but I still run into the redirect loop.
Right now am I running SilverStripe as a future platform for my website (the conversion process is ongoing) and haven't been able to move over the content since the redirect loop. The temporary site is available for viewing here: http://ren.cnwcentral.com/silverstripe/
Any pointers would be much, much appreciated!