Thanks for getting involved with this thread, jjulian.

I've reviewed our ongoing investigation about subscribers encountering a Error 500 - Internal Service error message when trying to process payments and can confirm Intuit's Product Investigations team is currently researching the behavior.

If you haven't yet, I'd recommend getting in touch with our Customer Care team and referencing the investigation's case number (INV-115470). They'll be able to pull up your account in a secure environment and add it to our list of affected users if necessary. This ensures you'll receive email notifications about any updates relating to our investigation.

They can be reached while you're using QuickBooks.

Here's how:
 

1. In your top menu bar, go to Help, then QuickBooks Desktop Help.
   
   
    
2. Click Contact Us.
   
   
    
3. Enter a brief description of the issue in our Ask a question (or tell us what's wrong) field.
   
   
    
4. Hit Continue.
   
   
    
5. Select We’ll contact you for a callback or Message Us to start an instant messaging session.

Be sure to review their support hours so you'll know when agents are available.

For the time being, the confirmed workarounds are to process payments through e-invoices you email to your customers for them to pay from their end, or you can manually process their payments yourself in the Merchant Service Center.

If there's any additional questions, I'm just a post away. Have an awesome Monday!