When I was offered the IMDB identifier suggestion for https://www.wikidata.org/wiki/Q43926 the preview link was to http://www.imdb.com/nm2312479/ instead of the correct http://www.imdb.com/name/nm2312479/. Because this 404s, users will be tempted to reject the suggested statement when it's correct, it's just the URL formatter that's wrong. After I accepted the suggestion, the URL was correctly formatted.
I suspect the issue is that https://www.wikidata.org/wiki/Property:P345 has multiple possible URL formats, which appear to be disambiguated based on regex matching against the identifier. The "nm" prefix should trigger the "name" form of the URL.