I'm also getting this with aadb2c auth. Downgrading to 1.1.7 fixes it.

  "auth": {
    "identityProviders": {
      "customOpenIdConnectProviders": {
        "aadb2c": {
          "registration": {
            "clientIdSettingName": "AZURE_CLIENT_ID",
            "clientCredential": {
              "clientSecretSettingName": "AZURE_CLIENT_SECRET"
            },
            "openIdConnectConfiguration": {
              "wellKnownOpenIdConfiguration": "https://tenant.b2clogin.com/tenant.onmicrosoft.com/B2C_1_sign_in/v2.0/.well-known/openid-configuration"
            }
          },
          "login": {
            "nameClaimType": "http://schemas.xmlsoap.org/ws/2005/05/identity/claims/name",
            "scopes": ["openid", "profile"],
            "loginParameterNames": []
          }
        }
      }
    }
  },

Edit to add:
A gotcha here is that you also need to change your registered callback URLs in your b2c configuration portal. if you change to aadbc like i did then your new callback uri should reflect that ie. https://mycoolapp.microsoftwhatever.net/.auth/login/aadbc/callback

Another gotcha is that it can take an arbitrary amount of time for Azure to update their redirect caches. This may lead one to an hour of work trying to figure out what they did wrong because aadb2c will report redirect_uri_mismatch errors even after you update your redirect configurations in the portal.

If you use User Flows then for some reason running the "Test User Flow" clears up this issue and your new callbacks will work. /shrug

I actually just ran in to this. This appears to be because of a difference in the expectations of the emulator's code and what the "documentation" suggest.

This issue is that these regex only parse for [a-z] and your custom provider is named "aadb2c" Which is what this documentation explicitly suggests.

if you change the name to something those regex can parse like "aadbc" it should work. You will need to change it everywhere in your staticwebapp.config.json.

ie:

{
  "auth": {
    "identityProviders": {
      "customOpenIdConnectProviders": {
        "aadbc": {
          "registration": {
            "clientIdSettingName": "AZURE_CLIENT_ID",
            "clientCredential": {
              "clientSecretSettingName": "AZURE_CLIENT_SECRET"
            },
            "openIdConnectConfiguration": {
              "wellKnownOpenIdConfiguration": "https://xxx.b2clogin.com/xxx.onmicrosoft.com/B2C_1_xxx/v2.0/.well-known/openid-configuration"
            }
          },
          "login": {
            "nameClaimType": "emails",
            "scopes": ["openid", "profile", "offline_access"],
            "loginParameterNames": []
          }
        }
      }
    }
  },
  "navigationFallback": {
    "rewrite": "/index.html",
    "exclude": ["/public/*"]
  },
  "routes": [
    {
      "route": "/sign-in",
      "redirect": "/.auth/login/aadbc"
    },
    {
      "route": "/sign-up",
      "redirect": "/.auth/login/aadbc"
    },
    {
      "route": "/logout",
      "redirect": "/.auth/logout?post_login_redirect_uri=sign-in"
    }
  ...
  ],
  "responseOverrides": {
    "401": {
      "statusCode": 302,
      "redirect": "/.auth/login/aadbc?post_login_redirect_uri=.referrer"
    }
  }
}

@aoscodes I can confirm this as well. After the 1.1.8 update, it seems that numerical characters are no longer valid and only [a-z] works. I was able to "fix" the issue by changing the provider key in my staticwebapp.config.json from auth0 to authzero and then navigating to /.auth/login/authzero instead.

Downgrading to 1.1.7 is the workaround for now I suppose. StaticWebApps still appears to work in production, just not the emulator.

@aoscodes for reference to others who use aadb2c as customOpenIdConnectProviders and see when developing locally after a redirect:

We couldn't find that page, please check the URL and try again.

I believe previously b2c was part of the docs with the naming of "b2c" as a proposed way to configure the authentication.
Example projects

Can confirm that this still happens with 2.0.1. Custom identity providers with numerical names are not supported. Affected by regex here. Solution was to rename the connector name from b2c to btoc 🙄