I believe there is a problem with the export service, which suddenly stopped working for us. The generated document returned is 0 bytes.
I think the easiest way to replicate is to look at the sample page:
https://docs.dhtmlx.com/gantt/samples/08_api/06_export.html
When “Export to PDF” is clicked, the following message appears:
“Internal server error. Error: Gantt to pdf. Error: Failed to generate pdf file”