I have a functioning login flow for my site via IdentityServer. When I test the flow on the CM server (but the frontend site) everything works ok. When I try the same thing on my CD server, I get:
Unable to reach an external identity provider.
IDX20803: Unable to obtain configuration from: '[PII is hidden]'.
No obvious errors are logged. It simply fails to find an IdentityProvider.
The Sitecore version is 9.1.1 and has been provisioned via Azure Marketplace.