Email Attachment stopped showing doctype information (shows blank PDF)

In a custom app, on_update event, in server script, I have a code which sends email in custom print format using (frappe.attach_print).

Used to work, suddenly stopped working in development environment (still works in production)

I wanted to send email on a different event, so made some code changes and noticed the attachment is blank.

Reverted all code changes, but attachment is still blank.

Not sure what to check in development environment (running VPS in VirtualBox, specifically for this project)

Any idea what might have caused it ?