After looked at the client's feedback, I think I know why it redirected to "Page Not Found". It is because the page 'contact' does not actually exist on live. So, the platform will try to redirect the page to standard 'No record' template but at the same tme, there is a Javascript to redirect the page to Login/Register page.
That is why the client saw "Page Not Found" happened intermittently. If the 'No record' finished loading first, they will see the "Page Not Found" but if the JS finished first, the page will redirect to Login/Register page.
Given that the page 'contact' has been changed to 'contactus' which it does exist and should reolve the page not found issue but it produces another issue after registered/logged in.
The page redirected to 'contactus' page instead. By the way, looking at the Javascript, this should be a correct logic because the JS has a logic to check if the URL contains keyword such as page, pcat, icat etc, it will redirect to that page/pcat/icat page instead of page/home.
However, if the client wants to take the customer to the page/home. I would suggest to change the rewrite rule again to:
http://topsourceportal.com/page/home?tmpgrp=golbon&assignusgrref=golbon&log=2
So that after logged in, it will take the customer to the home page correctly.
We can't make that URL change but I think the client can manage to change that URL again themselves.