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So I have tried this directly from the server, from a domain PC, from IE, chrome, edge, etc. And I always get the same result.
I have a CSV (I have tried both MS DOS and comma delimited). My CSV simply has the headers Username, password, display name, reply-to, first name, last name. Then 300 rows of values.
I go into Domain Settings > Accounts > Import from CSV and drop the file. I notice the NEXT never lights up unless I click the (?) question mark then immediately close the browser (not sure if that is a bug). When I click next it reads the headers from my CSV and I map them appropriately. Then I click SAVE and the import screen goes away but absolutely nothing else happens.