This is not a problem with our component. It is a problem with core Joomla. For Qlue Custom 404 to work fully, SEF URLs must be enabled.
We have now added a bold note on the overview to point users to the FAQs where this problem is described.
With SEF URLs disabled it is not possible to detect all page errors without hacking core Joomla. We prefer to suggest the far better solution of enabling SEF URLs.
Please read all documentation before reviewing - this is an exceptional, free extension that solves a long-standing problem in Joomla and knee-jerk reviews like this don't help other users.