While playing with the external secrets (changing some auth methods) and refreshing the web app, the /rest/external-secrets/providers endpoint has stopped working (simply returning 500). Now the web app returns nothing under /settings/external-secrets.
After investigating the code base, I realized that the list of providers were cached.
Something went wrong when saving them at some point and from then on, the cache was corrupted, leading to the aforementioned error.
A full reboot of the n8n deployments fixed the issue since this cache is only in-memory.