Hi Alexander,
I moved to new version 26.5 and here my results:
Please see:
Test data(input/output/QGIS)
5367-Gml GeoJson.zip (56.7 KB)
Thanks,...Bug 5367: Gml to GeoJson- Conversion succeeded, output in QGIS...(gml3.gml) is incorrect for conversion. No root element declaration...