Thanks for reaching out to us here in the Intuit Community, @Teryn.
It's possible the admin user is damaged, and that's why you're getting an unrecoverable error after signing in as the Administrator.
Discontinuation of the product is also one of the reasons since you no longer receive critical security updates. You may consider upgrading your file to the latest release to get rid of errors.
In the meantime, let's open your QuickBooks Desktop Pro 2015 and identify how many users are affected by using different login credentials. If the error only happens in the admin user, the credential might be damaged.
What you need to do is download and run the QuickBooks File Doctor to fix your company file. Once done, update and backup QuickBooks. For the detailed steps, please check this out for your guidance: Fix Company File and Network Issues with QuickBooks File Doctor, then follow steps 1 to 3.
If the same issue persists, you can perform other troubleshooting steps by going through this article: Unrecoverable Error General Troubleshooting Steps.
This issue may also occur in different situations, such as computer issues, missing Windows, or program codes that needs repairing. You can go through these links for additional information:
Let me know how everything turns out by leaving a comment below and I'll be get back to you.
