Error 500 on some orders

On my customers website they use this plugin. On most orders the plugin works perfectly. It also works on bulk and has done so in the past.

Since last week they have problems with generating shipping codes on certain orders. Reproducing it on a new order doesn’t give the error. It’s a error 500 that I can see in the console.

[Error] Failed to load resource: the server responded with a status of 500 () (admin-ajax.php, line 0)

In the dashboard you click the PostNL icon and a red X shows up next to it.