Error Message: Username (email@example.com) does not match with username (firstname.lastname@example.org).
This is because you have used a different user name on the Org tabs, then the one that you are authenticating with.
Let's say you are logged in Flosum org as the "Mary". You want to connect to Developer Sandbox as user "John".
- Login to Flosum as "Mary".
- Login to the developer sandbox as "John".
- On the orgs tab, enter the username for "John".
If the user name does not match in steps 2 and 3, then you will encounter the "user mismatch" error.