Applying an image transform returns a 404 cpresources URL
For what it's worth, I also had his issue, turned to imager, which gave me some more useful error messages as to my root cause of the issue:

You may need to make sure that the path to the asset file is complete, not just relative to the site base. So for instance, a base path of ./assets works fine for standard purposes, but appears no use for transforming images, you would need http://site/assets.

If your setting a base path in config, you can use something like

'environmentVariables' =&gt; array(
    'assetsBaseUrl' =&gt; '/assets',
    'assetsBasePath' =&gt; "{$_SERVER['DOCUMENT_ROOT']}/assets",
),