The server was unable to process the request due to an internal error

[fabhed]

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I am trying to authenticate with the ADFS method (username & password only) to another company's Sharepoint (company.sharepoint.com). It works fine when using an address from our organization after accepting an invitation to their Sharepoint Online. But when someone with their own company email address attempts the same, it always returns an internal server error:

Error: <s:Fault>
  <s:Code>
    <s:Value>s:Receiver</s:Value>
    <s:Subcode>
      <s:Value xmlns:a="http://schemas.microsoft.com/net/2005/12/windowscommunicationfoundation/dispatcher">a:InternalServiceFault</s:Value>
    </s:Subcode>
  </s:Code>
  <s:Reason>
    <s:Text xml:lang="en-US">The server was unable to process the request due to an internal error.  For more information about the error, either turn on IncludeExceptionDetailInFaults (either from ServiceBehaviorAttribute or from the &lt;serviceDebug&gt; configuration behavior) on the server in order to send the exception information back to the client, or turn on tracing as per the Microsoft .NET Framework SDK documentation and inspect the server trace logs.</s:Text>
  </s:Reason>
</s:Fault>
    at request.post.then.xmlResponse (/x/node_modules/node-sp-auth/lib/src/utils/AdfsHelper.js:31:23)
    at tryCatcher (/x/node_modules/bluebird/js/release/util.js:16:23)
    at Promise._settlePromiseFromHandler (/x/node_modules/bluebird/js/release/promise.js:512:31)
    at Promise._settlePromise (/x/node_modules/bluebird/js/release/promise.js:569:18)
    at Promise._settlePromise0 (/x/node_modules/bluebird/js/release/promise.js:614:10)
    at Promise._settlePromises (/x/node_modules/bluebird/js/release/promise.js:693:18)
    at Async._drainQueue (/x/node_modules/bluebird/js/release/async.js:133:16)
    at Async._drainQueues (/x/node_modules/bluebird/js/release/async.js:143:10)
    at Immediate.Async.drainQueues (/x/node_modules/bluebird/js/release/async.js:17:14)
    at runCallback (timers.js:794:20)
    at tryOnImmediate (timers.js:752:5)
    at processImmediate [as _immediateCallback] (timers.js:729:5)

What could be the reason for this? Should I instead adding the application as an add-in and authenticating a user with that?

[s-KaiNet]

I've never seen that before, so it's difficult to say what is the reason. I'd suggest using add-in only authentication (since that's SharePoint Online).