Javascript setInterval leads to "[HTTP/1.1 508 Loop Detected" on infinityfree?

The 508 error typically means that you’ve exceeded the limits of your hosting account, which is usually the Entry Process limit. You can learn more about that here:

You can use setInterval on your website if you want. But please make sure you’re not constantly hammering the server in the background to fetch new data. That will run through your account’s limits very quickly, and cause issues such as the one you’re experiencing right now. So you’ll want to make sure that the polling speed is reasonable, and that you’re not doing multiple polling loops for your account.