I had exactly the same symptom that I could not select the Page Content field of a Post or Page in order to enter text.
In my case the problem was that I had originally configured Options / General Options / WordPress URL with my numeric IP address and then switched it to a URL. I was still logging in with <IP_address>\wp-login.php rather than mydomain.com\wp-login.php. That confused WordPress, that sometimes it was referring to the IP address and sometimes the URL.
When I logged in with mysomdin.com\wp-login.php, then the problem was solved!
Cheers
— Mark
