I had the same problem. What ultimately helped was removing the cached information for the SSH Key. It took a while to locate the file, which depends on the version of Dreamweaver.
MODERATOR'S NOTE:
Be sure to Show Hidden Files & Folders within your file manager -- Win File Explorer or Mac Finder.
In general, you need to delete the line for the particular host(s) from the file ssh_hosts in the Dreamweaver area of ~/Library/Application Support/Adobe
Here are a couple of typical example locations:
~/Library/Application Support/Adobe/Dreamweaver CS6/en_US/Configuration/ssh_hosts
~/Library/Application Support/Adobe/Dreamweaver CC 2017/en_US/Configuration/ssh_hosts