Using "Customize form" button to customize list with InfoPath gives error: The SOAP message cannot be parsed (in SharePoint Online)

I have a list with just a few columns. When clicking on the "Customize form" button I the error: The SOAP message cannot be parsed.

This only happens in the SharePoint Online environment (I am using a trial to test out some things). When trying to do this on an on premise SharePoint 2010 environment then this works fine. I already added the comain to my trusted zone.