Filter editor does not show me any attributes to choose from

[wvdbee]

I've tried the Geoserver-Geostyler-add-on on a couple of layers: shapes, sql-querylayers, postgres-views. Alas I'm not able to select any attribute in the filter editor.
I've started with a simple copy of a polygon sld with only one rule.

1. Layer is connected to the preview. Standard demo layer with US-states.
2. Click on the filter icon
3. Try to select an attribute: no data-message
4. Maybe not related but it shows an 404-error, related to a font file that cannot be accessed.

Are there any installation or configuration options to set in order to get the filter editor working?

To be complete: rules without a filter do work as expected. Changes of additions of rules are previewed as expected.

Geoserver 2.26.1
Geostyler for geoserver 1.5.0

[jansule]

@dnlkoch could you take a look at this?