I now have reverse engineered the whole thing and finally got a json table like this:
[ [ { "waveSpawnTime":15000, "spawnpoint": [ 1, 1, 1, 1, 1, 1, 1, 1], "enemies": [ 4, 4, 4, 4, 4, 4, 4, 4], "hitpoints": [ 2, 2, 2, 2, 2, 2, 2, 2], "spawntime": [ 1050, 1050, 1050, 1050, 1050, 1050, 1050, 1050], "speed": [ 120, 120, 120, 120, 120, 120, 120, 120], "deathmoney": [ 5, 5, 5, 5, 5, 5, 5, 5], "rotationON": [ 1, 1, 1, 1, 1, 1, 1, 1], "scorePoints": [ 10, 10, 10, 10, 10, 10, 10, 10], "colorID": [ 14, 15, 16, 17, 18, 19, 20, 21], "resistance": [ 0, 0, 0, 0, 0, 0, 0, 0], "resstrength": [ 0, 0, 0, 0, 0, 0, 0, 0], "BarSpeed": [ 7, 7, 7, 7, 7, 7, 7, 7], "HP": [ 2, 2, 2, 2, 2, 2, 2, 2] }, { "waveSpawnTime":15000, "spawnpoint": [ 2, 2, 2, 2, 2, 2, 2, 2], "enemies": [ 4, 4, 4, 4, 4, 4, 4, 4], "hitpoints": [ 2, 2, 2, 2, 2, 2, 2, 2], "spawntime": [ 1050, 1050, 1050, 1050, 1050, 1050, 1050, 1050], "speed": [ 120, 120, 120, 120, 120, 120, 120, 120], "deathmoney": [ 5, 5, 5, 5, 5, 5, 5, 5], "rotationON": [ 1, 1, 1, 1, 1, 1, 1, 1], "scorePoints": [ 10, 10, 10, 10, 10, 10, 10, 10], "colorID": [ 14, 15, 16, 17, 18, 19, 20, 21], "resistance": [ 0, 0, 0, 0, 0, 0, 0, 0], "resstrength": [ 0, 0, 0, 0, 0, 0, 0, 0], "BarSpeed": [ 7, 7, 7, 7, 7, 7, 7, 7], "HP": [ 2, 2, 2, 2, 2, 2, 2, 2] }, { "waveSpawnTime":15000, "spawnpoint": [ 3, 3, 3, 3, 3, 3, 3, 3], "enemies": [ 4, 4, 4, 4, 4, 4, 4, 4], "hitpoints": [ 2, 2, 2, 2, 2, 2, 2, 2], "spawntime": [ 1050, 1050, 1050, 1050, 1050, 1050, 1050, 1050], "speed": [ 120, 120, 120, 120, 120, 120, 120, 120], "deathmoney": [ 5, 5, 5, 5, 5, 5, 5, 5], "rotationON": [ 1, 1, 1, 1, 1, 1, 1, 1], "scorePoints": [ 10, 10, 10, 10, 10, 10, 10, 10], "colorID": [ 14, 15, 16, 17, 18, 19, 20, 21], "resistance": [ 0, 0, 0, 0, 0, 0, 0, 0], "resstrength": [ 0, 0, 0, 0, 0, 0, 0, 0], "BarSpeed": [ 7, 7, 7, 7, 7, 7, 7, 7], "HP": [ 2, 2, 2, 2, 2, 2, 2, 2] } ] ]
Still get the error with the above code BUT I now have changed the code for decoding to this:
local path = system.pathForFile( "testwaves.json", system.DocumentsDirectory ) local file, errorString = io.open( path, "r" ) if not file then -- Error occurred; output the cause print( "File error: " .. errorString ) return WaveInfo\_G1\_S2\_World1 else -- Read data from file local contents = file:read( "\*a" ) -- Decode JSON data into Lua table local t = json.decode( contents ) -- Close the file handle io.close( file ) -- Return table return t end
And now it finally is working! 
Thank you for all your help! I really appreciate your help and fast feedback!
Have a great weekend!