This isn't the kind of experience we'd like you to feel, @Linkasink.

I appreciate you for getting back to this thread and performing the basic troubleshooting steps. Let me route you to the team that can help you further with your concern, 

I understand that you already called our Support Team, since your still unable to apply  customer credits to memo to your open invoices, I recommend reaching out to our customer support. This way, they can check your company file and trace the cause of the issue.

You can also check out our support hours to ensure that we address your concerns on time.

Feel free to visit this page to learn more about the A/R workflow: Get started with customer transaction workflows in QuickBooks Desktop

Let me know how it goes in the comment section. If you need additional assistance in doing your task with QuickBooks, don't hesitate to visit us back. We'll be happy to help.