I'm here to route you to the right support in fixing the error when your customers are trying to check out from your QBMS website, @CR62.
We have a separate and dedicated forum that handles QBMS concerns for QuickBooks Desktop (QBDT) Enterprise. I'd suggest posting your question from there. This way, our product developers can provide you an accurate solution to fix the error. They can also confirm if this is a QB or Magento issue. I'll guide you how.
- Go to this link: https://help.developer.intuit.com/s/.
- Select QuickBooks Desktop under What do you need help with?.
- Choose Ask a Question.
- Log into your existing Intuit Developer Account or create a new one.
- Enter "Fix QBMS Error: Unable to generate access token. Please check data processing your request." as your concern.
- Click Ask.
The screenshot below shows you the last two steps.
Lastly, I recommend visiting this website: Documentation Guide. It contains links about sending requests, managing transaction types, handling errors, testing different scenarios, and onboarding merchants through QBMS.
Don't hesitate to leave a comment below if you have other concerns. I'm always up for assistance, @CR62.