Hi Kwee,
I think it is looking for the file mentioned in the documentation here: https://docs.mendix.com/refguide/charts-configuration#4-chart-theming.
Do you have a chart included on some page, and is it only logging this error on that page specifically? Then it is probably trying to access that file from that chart.
Creating a file called ‘com.mendix.charts.json’ in your theme folder should fix your issue.