error 91 in Convert PDF using Task add in

I am setting up an automatic export PDF to a specific folder when a slddrw file is released. The tasks runs (I can see it in Task List in Admin tool), however, it wouldn't output a proper PDF. Instead I got a error message:

An unexpected error occurred while executing the generated script. Script syntax error?

Error number: 91

Error description: 'Object variable or With block variable not set'

Any idea how that happens? Thanks!

I am using:

EPDM 2015 SP4

Windows 7 on client

SolidworksSolidworks Pdm enterprise Pdm