You may encounter being stuck in a loop if attempting to log in to AppExchange when installing a package from a sandbox environment. The below error may appear, even if your credentials are correct:
"Please check your username and password. If you still can't log in, contact your Salesforce administrator."
Unable to Log in to a Sandbox with Correct Password
AppExchange App Trials: Introducing the New Simpler Trial Experience
Trailhead - AppExchange Basics
000381275

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