Hi @ste4nari, and welcome to the LE community forum
If you are doing external port 80 to internal port 8080, then:
Try using (with Apache vhost):
OR [if you haven't created, nor want to create, a vhost (in Apache) to handle port 8080]
AND
[that way certbot
can server the content needed on port 8080 for you]
See: User Guide — Certbot 2.7.0.dev0 documentation (eff-certbot.readthedocs.io)