Any chance there is a spacer or washer behind the cassette, against the hub, moving it closer to the dropout?
Once in a while a small washer may stick to the part it goes on and look like part of the "thing" it is against.

If the old cassette was working, the change you made is more likely something to do with the position of the part you changed. If you did not check this, double check even if you need to take the new cassette off. Do this before moving the derailleur over.