Hello. As some snapchat users have found out already, snapchat is increasing its security measures and thus falsely blocking some Android users:

This problem isn't GrapheneOS related, it also can happen falsely on the stock os.
This fix applies to the following error message: Due to repeated failed attempts or other unusual activity, my access to Snapchat is temporarily disabled .
You will be running an older snapchat version that might have unpatched issues. The version will be from February 2024.
Snapchat's 'official fix' (which won't work): https://help.snapchat.com/hc/en-us/articles/7012325477268-I-get-an-error-message-logging-in-to-Snapchat
Contacting Snapchat's support team won't help. You might complete a few messages back-and-fourth, but then their support will give up and won't respond to new replies (they literally say this lol)

How to fix this issue (for now)

Uninstall your current snapchat installation
Install snapchat version 12.67.xx. This is the latest version that will still work. Some newer snapchat versions will also let you log-in, but they will kick you out again after a few hours/days. A link to apkmirror, here. Older releases contain less features (such as the new high-res profile pictures and the new map, this version still has those luckily)
Log in normally
Don't update the app to a new version. In Aurora Store, you should blacklist the app in settings. I don't know if you can block the Play Store from updating automatically.

Alternative fix

Clear snapchat cache and data in settings
Create a new snapchat account
Enjoy. Be wary that creating multiple accounts on the same device will also result in this blocking issue again.

In the future this issue might be fixed and snapchat might roll-back the 'improved' security measures, but since snapchat is notiously bad at fixing issues or listening to its users, don't expect a better fix soon.

Hope this helps!