Hi Tobias,
I have found out that this is a known issue in qRules, and we don't have a fix scheduled for it. You'll find this in the User Guide:
When using the CopyTable in a SharePoint library or list, it will only work if the nodes match, which also means each column has to have data in it (no blank fields). The reason your fields can’t be blank is because when there is a blank field, no placeholder for that field is brought down in the XML, so it’s as if that node doesn’t exist.
We apologize that this is a current limitation in qRules.