The issue could also occours if you try to upgrade en instance with an install package and overwrite the web.config file with the new on from the install package.  If you then sett the new connection strings only, you could not connect or not connect well to the database about a wrong Maschine Key and missong objectQualifier and other databaseOwner as you used on the old version. 

Using the install package needed always a backed up web config file of the old version, and tranfering the both connection strings, the Maschine Key, and last but not least the objectQualifier and databaseOwner settings.