I went through the install process of 3.1. I made it through to the final page (http://localhost/boards/y1/install/index.php?mode=install&sub=final) which has a login button to proceed to acp (http://localhost/boards/y1/adm/index.php?i=send_statistics&mode=send_statistics&sid=93d02bf20612c4ede6428b4884697e14). When I pressed the login button however I was told:
After going back to board index, logging in and logging in to acp I was able to view acp just fine.
Purging the cache during install will prevent an incorrect (old) cookie name being read from cache and used for login after completion of the installer.