I think I might have an old version of the Paclet,
Your version seems fine, I have the same:
"Version" /. PacletInformation["SemanticImport"]
(* ==> "0.0.458" *)
Try removing "SemanticImport" from your machine and the re-run SemanticImport["ExampleData/50states.txt"]. If you are on Windows, it should be here: C:\Users\*username*\AppData\Roaming\Mathematica\Paclets\Temporary. Beware, AppData is a hidden folder.