I have Civicrm 4.7.20 installed on Wordpress 4.8.
When I create a new event or contribution the link that Civicrm creates does not work. What Civicrm creates as a link: http://[mysite]/?page=CiviCRM&q=civicrm/event/register&reset=1&id=7 This link loads my Civicrm base page, in this case my home page. If I change the base page to a different page then that page is loaded.
Note the &page_id=1811 parameter
As long as I specify the parameter &page_id=[any valid page] the link works, if I don't specify then the link loads the base page.
I tried disabling all other plugins and nothing changed so I think it has to be a Wordpress or Civicrm setting problem.