HiChooriang,

You are adding an aditional '[' bracket(see segment from recreated JSON below), which is throwing out your structure. I assume this is taking place in the ConvertToJson function that you have not provided.

"mat": [
[
{

The problem is likey eminating from either the function itself or at the point you are adding to your collections\dictionaries.
If i was to hazard a guess, i would suggest moving the section below inside the end if, you are probably populating a collection when nothing is found, based on your if statement.

MTdict.Add "merkmale", MKcoll
MTcoll.Add MTdict
MATcoll.Add MTcoll
Set MKcoll = Nothing
Set MTdict = Nothing
Set MTcoll = Nothing