Did you always upload the same file?

Yeah

As the error indicates, it seems that your CSV input seems to contain
characters that are not valid in XML. There are various ways to tackle
this; one looks as follows:

 (: interpret client data as Base64 :)
 declare
    %rest:path("/upload")
    %rest:POST("{$body}")
    function local:store-csv($body as xs:base64Binary)
  {
    (: replace invalid characters with a question mark :)
    let $input := bin:from-octets(
      bin:to-octets($body) ! (if(. >= 32 or . = (9, 10, 13)) then . else 63)
    )
    (: convert to XQuery Unicode string; convert to XML :)
    let $string := bin:decode-string($input, 'CP1251')
    return csv:parse($string)
  };

There may be easier solutions as well (I’ll give you an update once I
remember them ;).

Strange solution... Ok, I'll wait for a better one.

--

s0rr0w