[SalesForce] How to install a package to a sandbox while logged in via “Grant Login Access”

This is a problem with the salesforce session negotiation on sandbox orgs that use My Domain. There is no fix but a workaround is possible:

1. upon arriving at the login page, don't login, but view the page source,

2. locate the hidden input tag and value attribute by searching for "r=,

3. copy the whole string after the = and paste it into an RFC URL decoder,

You now have the redirect URL that was supposed to go back to the org. And, regardless of having other browser sessions open or not (even in a chrome incognito window) one will see that the pod is wrong, for example it may show cs30 or cs17 even if your pod is cs1 in actuality.

Replace the wrong pod with the right pod, and paste that amended URL into the browser for it to complete the dance and return you into salesforce.

As per How to Grant Login Access to a Salesforce Success Agent:

You can’t grant access to certain support organizations if your administrator has set up restrictions or a packaged application’s license prevents it.

Check that the user trying to grant login access has a license to the package application in question.