I'm not sure on Roku TVs, but ESPN+ is working fine on my Roku streaming box. I just tried it.
Are you logged in with your MyDisney account on the tv? Sorry if I'm asking questions that are too basic. That's all that's required to watch ESPN +.
What's the name of the stream you're trying to watch? From most of my checking on the ESPN website, it seems that the majority of Monday Night Football is not on ESPN +, so I am unsure.
If it was on ESPN or ESPN2, then do you have a cable/streaming provider that you are trying to authenticate with? I don't know how you could have seen it otherwise on your other devices. In that case, you can try calling ESPN (not ESPN +) to let them know of the issue. I don't know if they will get it fixed, but it wouldn't hurt.
I am unsure if maybe this authentication issue might have started because they were trying to fix the connection error issue. You could try chatting with ESPN support like I did, and link this thread. It might give you more ammunition that other people also have problems. Sorry if I am off base here with my response, and none of this helped you.
Here is a link that will get you to the ESPN chatbot (bottom right corner of webpage) if you choose to do a chat with ESPN. They should be able to connect you to an ESPN rep.
https://support.espn.com/hc/en-us/sections/115000951192-Authenticate-Sign-In