I, too, am attempting to use the Duplicator plugin to move a site to Dreamhost. I successfully get to the "Test" step, but on step 2, Remove Permalinks, I get the error: "Error establishing a database connection", with suggestions that I may have an incorrect user name or password, or that the database server may not be running.
However, if I look at the wp-config.php file, it appears to have the correct username and password, and I can log in to the database with PHPMyAdmin, and see that it is running.
I note that I get the same error if I try logging in to the WP site on Dreamhost.
Where might I look next to identify my error?
Ah--I found a peculiar value in the Hosts section of the wp-config file. I replaced that with the host name for the database, and now I'm past that error, and on to a new one...the login page briefly appears, then is replaced with a Error: redirect_uri_mismatch page. (I'm wondering if this is related to the Google login plugin.)