@Bas Lodewijks , Microsoft Q&A platform doesn't support Azure Certifications questions at this time.
Microsoft Certification Program is supported on their own forums.
To receive insights from the targetted audience on Certification related questions, I would suggest you to just re-post the question here :
https://trainingsupport.microsoft.com/en-us
See - Microsoft Certification help
In the interim, checkout the suggestions in this discussion thread: Error Code 90010703
"In filling out the information on your Certification Profile, please make sure to put all the details on your certification profile and make sure to update all special characters to roman alphabets/characters. E.g. (from ä to a) (, - .). Also, in the phone number field, you may move the first three digits on the area code to eliminate the error. "