Okay, I’ve tried porter 2.6 and it resulted in same UTF-8 error:
So far, I’ve followed this guide: Migrate a Vanilla forum to Discourse
Everything went well until this UTF-8 encoding error. Some people have resolved this issue. I’ve tried those, and it didn’t work out for me.
I tried @Nick_Chomey 's solution above; trying to enforce utf-8 encoding while reading the txt file but to my vain it didn’t work as well.