Magento 2 - Fix integration that throws decoding error

We go to 'system' 'integrations' here is a new integration we need for bookkeeping.

We use Magento 2.4.2 p2 with Varnish

We have all correct passwords, url etc.
Now when we try to 'Activate' the integration it throws very general error on backend "Sorry, something went wrong. Please try again later."

In the system.log we then find "main.ERROR: Cannot decode 6"

The integration manual is here: https://www-webwinkelfacturen-nl.translate.goog/handleiding-magento-minox?_x_tr_sl=nl&_x_tr_tl=en&_x_tr_hl=nl&_x_tr_pto=wapp

That page is in Dutch only so best to translate with google translate to your language.

Here you can find possible issues https://faq-webwinkelfacturen-nl.translate.goog/content/24/51/nl/het-lukt-niet-de-magento-webwinkel-te-koppelen-wat-kan-de-oorzaak-zijn.html?_x_tr_sl=nl&_x_tr_tl=en&_x_tr_hl=nl&_x_tr_pto=wapp

What do we need from you

1 - Debug the issue further
2 - Tell us what you want to test to get it silved
3 - Solve it

Please do not waste my time.
We will ask questions beforehand we give you any access.

Max time after starting project to finish is 3 days. If not finished then we stop.

Payment when done succesfull project.