If doppler login fails almost immediately with EOF errors on api.doppler.com but your network otherwise works, run this sanity check first: curl -v 'https://api.doppler.com/v3/projects'. If that looks healthy, the problem is almost certainly something intercepting your traffic locally, like a corporate network filter or proxy, which is exactly what caused this issue. The CLI polls /v3/auth/cli/authorize every 2 seconds while you complete the browser step, and a filter that kills repeated connections produces these EOFs. As a workaround while you sort the network, grab a personal token from the dashboard (Tokens, Personal tab) and load it with pbpaste | doppler configure set token. That bypasses the browser polling entirely.
_Source: gh:DopplerHQ/cli#362 (Doppler)_