Where does this error come from?

#1 hmm somehow it seems the datablock is not available.
be sure the script is correct and that you have the datablock available when in the mission.

#2 It seems to me like 'GameBaseData' is a hardcoded datablock which is automatically created when loading a misssion, in fact it is missing here.

greetings
Daniel

#3 GameBaseData is a c++ class one of the base root class's of anything that uses a datablock,
SpawnSphereMarker uses a datablock and there must be ones defined by the time you wish to add one to the mission.
in the script :
../server/scripts/markers.cs
you can find that the WayPoint and the SpawnSphere both share the same datablock MissionMarkerData which is derived from the GameBaseData class
I think getting this error means that you dont have SpawnSphereMarker filled out or something

datablock MissionMarkerData(SpawnSphereMarker)
{
   category = "Misc";
   shapeFile = "~/data/shapes/markers/octahedron.dts";
};

you are getting this error when adding one to the mission?
do you have one in the mission?

#4 The datablock is added, but when the game tries to add a spawnsphere or to spawn the player, it is missed.

greetings
Daniel

#5 Now I got it.

The datablocks are loaded before the game was created. I ask myself why this did not cause an error.

OK, now I load the datablocks in the createGame() function and now it works fine.

greetings
Daniel