As quoted from the linked post.
It looks like you’re part of one of our experiments. The logged-in mobile web experience is currently unavailable for a portion of users. To access the site you can log on via desktop, the mobile apps, or wait for the experiment to conclude.
This is separate from the API issue. This will actually BLOCK you from even viewing reddit on your phone without using the official app.
Archive.org link in case the post is removed.
Since I’m using the app right now, I’ll have to recite this from memory. But at the bottom of the screen you should see a button second from the left three lines.
Click on that and search for a group in the top portion. Click on the group once found , and there there is a subscribe button. As of yesterday cliking it appears to do nothing. In fact it does work. Exit the list and go back in, you will be subscribed.