PHP error on submitting form: Call to a member function: getProductId() on a non-object in SoapClient->__soapCall()

This is the full error:
SoapFault: Call to a member function getProductId() on a non-object in SoapClient->__soapCall() (regel 105 van /vendor/finlet/flexmail/src/FlexmailAPI/FlexmailAPI.php).

I created a flexmail trial account for development purposes and with that account everything worked fine. However, as soon as I used the user_id and user_token our client has provided I get that error.

When I look at the settings page here /admin/config/services/flexmail I do see the mailing lists for that account, so both the user_id and the user_token I received from the client are correct.