Show Details information could help to reveal an ORA- error --
guess it could be something like ORA-01843.
Well, it's a known behavior of Oracle DBMS --
implicit CAST'ing a STRING (like "4/15/2015") to a DATE
could result to a valid date -- when using "mm/dd/yyyy" mask --
or give an error -- when using "dd/mm/yyyy" mask.
Which one is used depends on your Oracle environment.
Using DATE("2015-04-15") is a safe way of doing in Tableau.
Most DBMS engines cast this string correctly as "yyyy-mm-dd".
Hope this could help.