Running into the same issue, though I can't tell since when this is the case as I never had snapchat on graphene OS.
I did reach out to snapchat's support and they mentioned two points:
If you are using a VPN, we recommend that you switch it off. If you are not using a VPN, try switching to a different network (changing from WiFi to a mobile network, switching to a different WiFi network, etc.).
If you are experiencing issues creating an account and have a rooted device, you may want to try unrooting your device, removing any exploitation software, and reinstalling the app before creating an account.
Rooted devices circumvent security defenses built into the mobile device, exposing your Snapchat account to additional risk and may not be supported.
The network thing didn't make a difference for me with/without a VPN, wifi or my cellular connection, neither did installing version 11.95.0.39.
I suspect this is related to the second point about "rooted devices". To me this sounds like they introduced some kind of detection and just throw you the "SS03" or "something went wrong".
The exploit protection compatibily flag also didn't change it.