Hi,
Hard to say why that would happen.
The error message is actually from MyParcel's servers side.
And apparently, it's because there is something wrong with the information sent to it, but it doesn't say what...
I checked the information that isn't blurred on your screenshot, and it all looks fine. It's actually a pretty standard request for MyParcel... standard shipping type, standard delivery, no option ...
So maybe the issue is linked with the user address ?
For example, it is trying to do a local (NL) shipping but the address is in another country ? Or the post code is not a valid one ? Or the address doesn't exist ? But with the address blurred I don't know what could be problematic there.
The best would be to contact MyParcel's support with the whole message not blurred so that they can check what's going on. They should be able to better answer you if you are able to get to a tech support on their side as they'll know what could generate that error on their server.