4)  Any dn or rdn that contains characters other than those
          defined as "SAFE-UTF8-CHAR", or begins with a character other
          than those defined as "SAFE-INIT-UTF8-CHAR", above, MUST be
          base-64 encoded.  Other values MAY be base-64 encoded.  Any
          value that contains characters other than those defined as
          "SAFE-CHAR", or begins with a character other than those
          defined as "SAFE-INIT-CHAR", above, MUST be base-64 encoded.
          Other values MAY be base-64 encoded.

      8)  Values or distinguished names that end with SPACE SHOULD be
          base-64 encoded.

SAFE-CHAR                = %x01-09 / %x0B-0C / %x0E-7F
                           ; any value <= 127 decimal except NUL, LF,
                           ; and CR

If you are sure there is nothing but SAFE-CHAR, check for space or and non-ascii charset. 

Sometimes there are invisible characters in eg. latin1 that will "hide" in a charset capable editor.


(characters other than US-ASCII would be encoded also - view data with a dumber text editor)

Cheers
Brett