HighCharts Export server "invalid" input data - highcharts

Question

I'm testing the HighCharts Export server with a load of 1000 requests. All the requests are the same so I should receive the same output for the 1000 tests. The JSON request is the following:
{
"type": "png",
"scale": 1,
"infile": {
"chart": {
"backgroundColor": null,
"width": 1000,
"height": 200,
"style": {
"fontFamily": "Arial"
}
},
"title": {
"text": ""
},
"plotOptions": {
"bar": {
"pointPadding": 0,
"borderWidth": 0,
"dataLabels": {
"enabled": false,
"fontWeight": "normal"
}
}
},
"xAxis": {
"tickWidth": 0,
"gridLineColor": "#E2E8ED",
"categories": ["Jan", "Fév", "Mar", "Avr", "Mai", "Juin", "Juil", "Aoû", "Sep", "Oct", "Nov", "Déc"],
"labels": {
"enabled": true,
"style": {
"fontSize": "14px",
"color": "#52626F"
}
}
},
"yAxis": {
"min": -4,
"max": 10,
"tickInterval": 2,
"labels": {
"format": "{value}%",
"style": {
"fontSize": "14px",
"color": "#52626F"
}
},
"title": {
"text": null
}
},
"series": [{
"name": "xxx",
"type": "column",
"color": "#C20044",
"pointWidth": 43,
"data": [3.65, 2.70, 1.40, -0.85, 0.25, -2.65, 1.40, -3.40, -1.30, 3.45, 1.35, -2.35]
},
{
"name": "xxx",
"type": "line",
"color": "#52626F",
"lineWidth": 1.5,
"marker": {
"enabled": false
},
"data": [3.65, 6.35, 7.75, 6.90, 7.15, 4.50, 5.90, 2.50, 1.20, 4.65, 6.00, 3.65]
},
{
"showInLegend": false,
"lineWidth": 0,
"data": [0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0],
"marker": {
"enabled": false
}
}
],
"legend": {
"enabled": true,
"layout": "vertical",
"align": "right",
"verticalAlign": "middle",
"margin": 65,
"itemMarginBottom": 6,
"itemMarginTop": 6,
"symbolRadius": 0,
"style": {
"fontSize": "14px",
"color": "#52626F"
}
},
"credits": {
"enabled": false
}
}
}
And the output is this chart:
There is nothing wrong with the output itself but after a while the HighCharts Server returns an error which says that the input data was invalid.
[error] work 9fa383e8bd0f44f3a1e50dc95b13df71 could not be completed, sending: error when performing chart generation: please check your input data. However the request hasn't been changed. Mostly the error occurs when we're halfway.
I run the export server using the following command highcharts-export-server --enableServer 1 --port 8080 --workers 20 --workLimit 200
A worklimit of 200 seems me a good number because we cross this amount of requests.
Does anyone have an idea why it suddenly doesn't want to process the request and says I need to check my input data?
Btw. The export server is running in a docker container, but I think that does not cause the problem.