Link to User record, via View Contact -> Actions -> User Record gives Page Not Found (404). Is this a bug?
I just tested on a site. Clicking the link as described above i get sent to 

user/2&amp;cid=39941

and that page shows.

BUT

if instead I just click on the Contact ID / User ID |
39941  / 2 that sits just under Tags on the Contact Summary screen

 

I get sent to 

user/2 and that sounds like it might work in your set up

Does your Summary Screen not show that? (ie I wonder why you are not using that rather than going via Actions which is an extra click)

I can't explain why the link with &amp;cid=YYY is failing for you