Whenever problems occur during the creation of print outputs, it can be helpful to check the log file generated by the VertiGIS Studio Printing Engine. When a print is executed with VertiGIS Printing, a scratch directory is created under the following path:
"...\arcgisserver\directories\arcgisjobs\vertigis_utilities\vertigis_printing_tools_gpserver\<jobnumber>".

 

In this folder you can find the web_map_as_json as a .json-file.