If the search usually works, but fails when searching by a specific ISBN/ISSN/OCN, there is a likely an issue with one of the bibliographic records in the search results. Search WorldCat.org by that ISBN / ISSN / OCN and look for unusual hex characters or excessively long 5xx fields. Edit the record to get rid of the problem characters and retry downloading the record. If problems persist, contact OCLC Support with the ISBN/ISSN/OCN of the search, any OCNs you've already altered and your library's OCLC symbol.