Okay. I found a SUPER simple resolution. After you enter the account nickname, account number, zip code, and country - the next pop up is the log in page to allow ShipEngine access. Do not enter your email. Maximize the pop up and click on the "Log In" button in the upper right hand corner of the window. Enter your UPS or UPS My Choice username and password. 

This worked for me after hours of scouring the webs for fixes. I updated Quickbooks Desktop (2024), changed crazy registry keys, tried running as Admin.... you name it. This was my last ditch effort before going Office Space on our PC. Thankfully, it was spared the baseball bat.