Hey there! This is almost always caused by an issue with a broken redirect in a .htaccess file. Apache has the LogLevel configuration option set to "warn" by default, but you can change that in WHM >> Apache Configuration >> Global Configuration to Debug if you wanted to see more errors.
It is critically important that you only leave that option enabled while you troubleshoot this particular issue as it will log everything Apache is doing, and could potentially quickly fill a disk partition on an active server.