Thanks for your Reply :)
We found the following entry:
The following query failed: OpenWorkbook (User: 0#.w|mydomain\farmaccount, Form Name: Template, IP: , Connection Target: , Request: etc.)
Exception Message: The remote server returned an error: (500) Internal Server Error. You do not have appropriate permissions to create a new workbook in the specified location.
I think the claims based authentication method in Sharepoint 2013 could be the reason for my problem.
Found this on the internet:
https://blogs.msdn.microsoft.com/russmax/2012/08/17/want-to-call-sharepoint-2010-web-services-within-browser-based-infopath-2010-forms/
Now trying to figure out an easier way to get my form working. Any further ideas would be much appreciated :)