Finally got this to work! It seems that the nodemailer plugin has a dependency on node v 6.0.0. As soon as I changed the dependency to a compatible version the problem was solved.
Hope this helps someone else facing the same issue.
Are there plans to update this for 2.0? The formatting in this one looks easier for users than the formatting in nodebb-plugin-btn-spoilers, so I would prefer to use this.
I will need upload functional in future plugin.
Is there any Upload API already in NodeBB ready for plugins use?
I would like to use ready to use solution, if any, before I will do it myself 😉