I've found an additional solution to this issue for those searching, after spending some time with Regions Technical Support. It is indeed an issue on their end - not QuickBooks Online.
There is a pop-up within Regions Online Banking that will ask you to update your personal information. Which is what is causing the error.
1. Use Google Chrome if you're not already.
2. Disable any pop-up blockers you have.
3. Log-in to Regions on a desktop or laptop (not an iPhone) and set your DEFAULT ONLINE account as your personal account.* (You can find the steps to do that here: https://www.regions.com/help/online-banking-help/getting-started/starting-online-banking/how-to-set-...
*My business and personal accounts are linked with one ID. The pop-up only appears if you have defaulted to your personal online.
4. Log-out and Log back in and the pop-up should appear, update your personal info.
Regions weren't aware of this particular solution. I did this and the Regions rep actually told me I was one of the only ones since the 8th to successfully fix this issue.
Hope this helps!
Everything has now been working properly.