From notes on ldif syntax :
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