peppe I have had the same or similar issue. And I have been able to both log in and be blocked from logging in from two or three different GrapheneOS devices. Two Pixel 5As and one 8A. One 5A broke, and I had a spare, unused 5A that I would keep around for parts, or as a spare phone dedicated to travel and other misc uses. My primary 5A was malfunctioning, so I was trying to migrate to the unused/new Pixel 5A that I have had laying around for a long while. Both phones fully updated. With the new 5A not working with my snap logins blocked, I decided that with support ending for the 5A, I will get an 8A ordered in.
This is my observation. For phones that have already had snapchat logged into it in the past, I am able to log in just fine. With my stock-android Google Pixel from 2016 or my old, malfunctioning GrapheneOS 5A that I have used for a while, I can log in and out without issue.
The new Pixel 5A blocks my logins with the same error you guys describe. Regardless of being on a fresh user profile, play services installed or uninstalled, or a VPN service being enabled/not. I figured it could be a device ban of sorts. I installed a fresh install of Graphene on the 8A this morning. Logins are blocked.
I ran across a reddit thread about how Snapchat checks for Roots upon login and the workaround was downloading the
Snapchat 11.95.0.39 APK. Users there say that Snap checks for root upon login, but you can update the app afterwards. Although their advice was not Graphene-specific. I installed it on the 8A, which was blocking my logins prior, and now I can log in just fine.