Marco just grabbed my laptop to see what can cause this error, but I don't immediately see what's going wrong here.
The only thing I have spotted so far is the following;
X-Bunq-Client-Request-Id: X-Bunq-Client-Request-Id: 1535282173.19266-a230b06ba65fXXX
I think it is a copy-paste error, but just double-checking, are you sending your request like this?