Check character set of Database which export dump file has been already taken.
You check the character set with below query.
select * from v$nls_parameters where parameter in ('NLS_CHARACTERSET','NLS_LANGUAGE');
Also check the character set of DB which needs to be imported.
Check whether the characterset conversion is possible or not.
Sometimes a corrupted dumpfile cause this error.Suppose if you are trying to import using IMP from a EXPDP(Data pump Export)dumpfile Cause this error .