A debug mode was created for the versions as from 1.63.1.

If you are having trouble printing your bpost's orders, you can enable debug mode to find the cause.

Debug mode is only visible in Orders > bpost > admin and can be made visible by appending “&debug=1” to address bar.

From there, it can be made to stick by checking the box. (Unchecking turns it off)

Important : When  debug mode is on, every call is intercepted and logged, meaning it takes a big hit on resources if  you let it arbitrarily left on.

So don't forget to deactivate it as soon as you debugging is completed.