I got my process working by selecting 2.0.0 as a used WFS version in the FME WFS Reader.
FME forms then the following url for DescribeFeatures request:
https://www.company.com/geoserver/customer_c/wfs
?SERVICE=WFS
&VERSION=2.0.0
&REQUEST=DescribeFeatureType
&TYPENAMES=customer_c:Data
&NAMESPACES=xmlns(customer_c,https://company.com/geoserver/customer_c)
&OUTPUTFORMAT=application/gml+xml; version=3.2
I still don’t know/understand where the problem actually was. It might even be somewhere outside GeoServer. However, it works now, and my working hours are limited, so I can’t dive that deep into this.
Thank you all for your help!