VAT-number not validating, even though it is correct

We are using EU/UK VAT Validation Manager for WooCommerce and are encountering some issues.
Some of our customers VAT numbers get declined, even tough these are correct VAT numbers. The message beneath the Vat-number field even indicates the number as valid. When trying to pay however, the bigger error message box states that the number is invalid.

Myself and another developer have tried to recreate this issue, to no succes. However, we keep getting complaints from customers encountering this issue. Here is a quote from a customer experiencing the issue:

“The biggest problem was entering the VAT number.
When we entered it, the message “Add an official or recognized VAT number” appeared.
(Even though this was the correct number, of course).
Eventually, it worked.”

We’ve been trying to get this issue resolved for a while now, any insight or help would be very welcome.

Thank you. Best regards