If you have MySites and a Search Center running in your farm, you may have overlooked a not-so-obvious configuration setting. When setting up MySites for a User Profile service application, your Search Center URL must point to the "Pages" folder where the Search Center publishing pages are located instead of the Search Center web URL. Otherwise, a People Search from a MySite will forward the request to to
(which doesn't exist) instead of
which is where the PeopleResults.aspx page is actually located.
TLDR: Use http://[searchcenter]/Pages/ for your Search Center url in UPA MySite Settings.