We got a call for this issue.
The client upgraded from QuickBooks 2012 to 2015 on two machines. After that they were unable to open their two company files.
I upgraded the copy on their server. That wasn’t a total fix.
I found the admin account in some documentation. Didn’t know the password but was able to reset it. After that we opened the company file with the admin account. The file updated itself and after that the QuickBooks file was good to go.
I hope this help someone out there….