Doing my federal (US) taxes this year, I was finding myself unable to create an account on the Free Fillable Forms site. I kept getting kicked back to the start after filling out and submitting my information. This Reddit thread led me to figure out that it was related to my T-Mobile Home Internet. I turned that off and turned on my phone hotspot and used that to get in. My phone is also T-Mobile, but apparently different enough to not have that issue. Later, I was able to get back in with my Home Internet, so it seems to be just related to something on that initial registration process.