'You are not a member of this organization' error when accessing D365 instances (like through an AppSource Test Drive) that are not in home tenant

Note: Please note that the response below was carried over from a previous marketplace for partners community discussion. Original authorship has been removed.

I had this error come up for my own account (system administrator). What solved it for me was to clear my browser cookies and restart the browser. I think Microsoft 365 gets confused sometimes when you have multiple signed in accounts in the same browser. Just thought I'd share my observations. Maybe someone else will find this helpful.