@arev Did you try exporting them as Alembic files? That worked for me in the end. I don’t think the exporter plugin from Sketchfab will work in a alpha stage application like Blender 2.8. The problems with I had with exporting my file (see a few posts before yours) had more to do with data corruption, if you ask me, then a non working exporter plugin.
Both the Collada and Alembic formats are supported by Sketchfab, but only the last one worked for me.
