I can't logout. When I click the logout link, I get a message saying "You Have Logged Off Successfully!". Then it opens the vsadmin/admin.php page, and repeats this loop.
I didn't have this problem before I updated to 7.3.8 from 7.3.7.
Also caching can make it appear as if the login isn't working correctly as you are always shown the same page which is a cached page. You can check out the headers by checking out one of the regular store pages using the following link http://tools.seobook.com/server-header-checker/