Please see this thread about error #1500:
Quick summary: When you submit new link to Facebook, Facebook takes a look at this link and indexes it. When Facebook servers are overloaded they can’t index it during the request time. Facebook throws one of the errors mentioned above. Facebook still indexes the link, so the second try couple minutes later usually goes without any problems.
#1500 Error was very common during summer 2013, then it become better. Recently (Jan-Feb 2014) the same problem resurfaced as “An unexpected error has occurred” or “This link could not be posted” error.
Yes we do have a pretty good idea what this problem is about, but we can’t do anything with this. This problem is on Facebook side and they need to fix it. The problem is that Facebook servers are overloaded and if your site is not fast enough for them to get all necessary info in time, so they just trow it with #1500 error. They need to upgrade their infrastructure and increase the timeout. Until that you can only try to set the correct OG: tags, speed up your site somehow or find a faster hosting.
Update Jan/Feb 2014: – It looks like “Attached” posts are more likely to be affected by this then “Shared”, so try to switch the format.