I had/have the same issue with QB2018 Premier. I don't think it's a permanent fix but I used the Quickbooks Tool Hub Utility and had it run through the installation issues (Quickbooks Install Diagnostic Tool). This worked but I'm still having some issues. The can't open the register issue happened again so I ran the tool again and I'm able to see the registers but I've also had QB crash several times for me today.
Also, QuickBooks Desktop 2012 that you're using is no longer supported, this may cause some functionality might not work properly. To run your accounting needs efficiently, you can upgrade your QuickBooks Desktop to the latest version when you're ready.
Let me know if you have any other questions. I'd be happy to help. Take care always.
I'm experiencing the same thing with QBPro 2018. It just started today. It is the same on both office and home computer. I tried repairing from control panel but no success. I finally uninstalled and reinstalled QB and it worked. But when I updated QB after reinstalling it wouldn't work. I also tried the Quickbooks Tool Hub as suggested but no success. This is very frustrating.