(I also posted this to this forum.)

How do I eliminate or work around a 500 error preventing me from using Lightning components on a VisualForce page in a sandbox? Identical code works in a dev org but fails in a sandbox.

Note: In the sandbox I used, the "My Domain" feature is in a "Ready for Testing but Hidden from Users" state, whereas in the dev org I used, the "My Domain" feature is in a fully deployed state.

Only mentioning My Domain for good measure, since this document:

Finally, you don’t need to enable My Domain to use Lightning
components in the following contexts.

•  Lightning components with Communities in Community Builder
•  Lightning Components for Visualforce

Copious details below.

Inputs
Sample code taken verbatim from Lightning Components Developer Guide.

lcvfTest.app

<aura:application access="GLOBAL" extends="ltng:outApp"> 
    <aura:dependency resource="ui:button"/>
</aura:application>

lcvfTest.vfp

<apex:page >
    <apex:includeLightning />

    <div id="lightning" />

    <script>
        $Lightning.use("c:lcvfTest", function() {
          $Lightning.createComponent("ui:button",
          { label : "Press Me!" },
          "lightning",
          function(cmp) {
            // do some stuff
          });
        });
    </script>
</apex:page>

Results
Dev org succeeded but Sandbox failed.

Dev Org
Renders a button, just as described in the documentation.

Sandbox Org
No Button is rendered. The browser's debug console reveals a request which fails with a 500 error. 

(Note: In the following output, hash-like values were replaced with fictitious ones, since I do not know whether the hashes contain private information. Domain and sandbox names are also fictitious.)

The URL of the request which triggered a 500 error was:

https://my-custom-domain--my-sandbox-name.cs2.my.salesforce.com/l/%7B%22mode%22%3A%22PROD%22%2C%22app%22%3A%22c%3AlcvfTest%22%2C%22fwuid%22%3A%22FNQR24ngqrHzyBZ_Nh3ghS%22%2C%22loaded%22%3A%7B%22APPLICATION%40markup%3A%2F%2Fc%3AlcvfTest%22%3A%22N7gJLRFbk4dlneywPZJt2f%22%7D%2C%22requestedLocales%22%3A%5B%22en_US%22%2C%22en%22%5D%2C%22ls%22%3A%22YN%22%7D/inline.js

Opening this URL in its own browser tab yields the following content (indentation added by me):

aura.error(
{
    "exceptionEvent":true,
    "event": {
        "descriptor":"markup://aura:clientOutOfSync",
        "eventDef": {
            "s":1,
            "v": {
                "descriptor":"markup://aura:clientOutOfSync",
                "type":"APPLICATION",
                "superDef": {
                    "s":2,
                    "v": {
                        "descriptor":"markup://aura:applicationEvent",
                        "type":"APPLICATION",
                        "xs":"G",
                        "attributes":{}
                    }
                },
                "attributes":{}
            }
        }
    },
    "defaultHandler":"function() {$A.clientService.setOutdated()}"
});

The large URL-encoded portion of the URL above decodes to this (indentation added by me):

{
    "mode":"PROD",
    "app":"c:lcvfTest",
    "fwuid":"FNQR24ngqrHzyBZ_Nh3ghS",
    "loaded":
    {
        "APPLICATION@markup://c:lcvfTest":"N7gJLRFbk4dlneywPZJt2f"
    },
    "requestedLocales":["en_US","en"],
    "ls":"YN"
}