Store module "Mail Merge Reports" - template upload fails

Hi,

Does anybody here have a working installation of Mail Merge Reports module?

I am trying to get it operational on Ubuntu 16.04 LTS. Unfortunately, I can’t get it to work properly: I can create template records itself, but any document template file uploads fail. The record saves, but doesn’t upload the docx template file (the filename field remains empty in DetailView).

The developer (Izertis) suspects a permission issue on server-side directory “document_templates”. I am looking for advice on how said permissions look in a working installation.

Regards,
Wolf