Have any of those having this problem actually tried contacting Apple Support to get them to help you resolve the issue?
You get 90 days free Applecare Pro Apps support when you purchase LPX so make the most of it....
In respect of this specific problem, in every case i have had to deal with, in regards to a few of my clients who had this problem both in LPX and formally in LP9, the cause has been found to be either an issue with the client's Internet Provider, their home network or a specific issue with their Mac... usually related to permissions or migrated accounts... and not actually a problem with Logic Pro itself. Once the 'outside' issue has been fixed, Logic Pro's content downloaded without issue...... as it does normally for the vast majority of Logic Pro users.