I am running DNN 5.1.1 and asp.net 4.  I recently updated from a much earlier version to 5.1.1 and a few days ago took the further step of changing to asp.net 4.  Following the latter, the site exhibited some strange behaviour which I have  mostly eliminated by modifying web.config after searching the net, including these forums, for ideas.  The site is running fine except that 

(a) the Register link takes you nowhere - although the javascript call shows in the line at the bottom of the browser but when the link is clicked basically nothing happens and 

(b) the Login link works as far as taking you to the login form and allowing you to enter user ID and password - it even identifies if an incorrect id and password are entered.  However, when the login button is clicked you end up back on the home page and not logged in.  Understandably this makes it rather hard to administer the site!

Skin is modified Minimal Extropy.  I did not not set up the site but have administered it for some time but am still learning about the finer points of DNN's internal workings