I have found a likely explanation in Reading GDML File in ROOT 6.25 - #4 by FFoll but I still don’t understand why the first case (using box) works and the tesselated solid does not work, perhaps @agheata can shine some light on this topic ^^. To me this looks like a bug in the parser.
My goal would be to have each of the gdml module files be readable on their own (thats why they have a world volume and so on) and also be able to be incorporated into a mother gdml. I thought this was possible since I can do it with the box example.
I have tried to name the vertices in the file as suggested in the linked reply (such as vertex1="cube_v0_Volumes/child.gdml") but it gives out the same error. I really don’t want to place all the definitions inside the mother file since that would defeat the purpose of splitting the file.