I think you have a bug in your PHP Source Code. So the CMS can't create dev/build. You can post your source code, so I can help you. Sometimes it's a simple syntax error in the file, and this can make a SilverStripe crash without any error code.
You can try to change your rights of your SilverStripe folder. Set the folder into 777 (only for test). Maybe one or more files don't have enough rights, so SilverStripe can't find /dev. I hope this is the solution of your problem.
I've tried your advise setting rights to 777 without having success.
After that I deleted the old silverstripe installation and set up a new one. After the install process finished I think I've got a hint, why /dev/build?flush=1 fails.
I got the following message, which I didn't realized last time. Friendly URLs are not working. This is most likely because mod_rewrite isn't configuredcorrectly on your site. Please check the following things in your Apache configuration; you may need to get your web host or server administrator to do this for you:
* mod_rewrite is enabled
* AllowOverride All is set for your directory
As mod_rewrite is enabled on the server (works fine under drupal) I've changed the virtual host configuration by adding
Trying to reinstall silverstripe (reloading the "Installing SilverStripe..." - page) to see wether the warning appears or not, the warning persits.