I appreciate the effort you've made in attempting to resolve this issue, Markus, like trying with a second account and providing a screenshot. I'd be glad to guide you on where to find the best help available to successfully integrate UPS in your QuickBooks Desktop (QBDT).

Since you have verified that the account's postal code and account number are accurate, I recommend contacting UPS customer support for specialized assistance. They are well-equipped to provide comprehensive guidance and help troubleshoot the error message you're encountering, ensuring the seamless integration of UPS within the system.

Moreover, for additional information on the steps and frequently asked questions about UPS, please feel free to explore this article: Intuit QuickBooks Shipping Manager.

If you need additional assistance or have further questions about the integration process, please don't hesitate to get in touch. The Community space is always here to provide you with guidance and help resolving any issues you might encounter every step of the way.