Fast and Easy Checkout

[numinix]

Try disabling your SEO URLs mod and if that corrects the issue, then re-enable and try disabling it only for the checkout pages. How you achieve that will depend on the mod you are using.

If that's not the problem, have there been any customizations to your shipping modules? The checkout will redirect back on itself to select a shipping method automatically if one isn't already selected. If it's unable to make a selection, this could cause the infinite loop. Normally if there are no methods, it'll display an error message. But if there are methods and one of those methods isn't coded properly, it can lead to this bug.