Upgrading Drupal 7.43 to 8 error: Source database is Drupal version 8 but version 7 was selected
I managed to solve it. 
It was caused be an update on PHP, that caused blank pages or “white screen of death” (WSOD) in the admin, too. 
After updateing to 7.59, everithing was OK.
This article helped me: https://www.drupal.org/node/158043