This is really strange! Just wanted to update von 4.4.3 to 4.4.5 and got exactly the same issue again! 
Forward80To443 is set to true.
Edit: I tried it to set it to false and et voilá, it’s working again. 
If I enable it again and restarting mattermost I got the same problem again.
Just for the record: I use the provided configuration for nginx with Let’s Encrypt.