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.
The sad thing is, the official Reddit app was not too bad at the beginning. I even used it when it just came out. But in the last few years, every change that they implemented seems to have been a negative one. The ads were annoying, but the tiktok-style video viewer was the last straw for me.