SET TERM ^ ;

CREATE OR ALTER procedure sp_insupd (
    iaction varchar(3),
    iusuario varchar(20),
    iip varchar(15),
    imodulo varchar(30),
    ifieldsvalues varchar(2000),
    iwhere varchar(1000),
    idesclogs varchar(200))
returns (
    oid integer)
as
declare variable vdesc varchar(10000);
begin
  if (iaction = 'ins') then
  begin
    vdesc = idesclogs;

    /*** o erro está na linha abaixo ***/
    execute statement 'insert into '||:imodulo||' '||:ifieldsvalues||' returning ID into '||:oid||';';
  end else
  if (iaction = 'upd') then
  begin
    execute statement 'select '||:idesclogs||' from '||:imodulo||' where '||:iwhere into :vdesc;

    execute statement 'execute procedure SP_CREATE_AUDIT('''||:imodulo||''');';

    execute statement 'update '||:imodulo||' set '||:ifieldsvalues||' where '||:iwhere||';';
  end

  insert into LOGS(USUARIO, IP, MODULO, TIPO, DESCRICAO) values (
  :iusuario, :iip, :imodulo, (case :iaction when 'ins' then 1 when 'upd' then 2 end), :vdesc);
end^

SET TERM ; ^

...
execute statement 'insert into '||:imodulo||' '||:ifieldsvalues||';';
...