"You cannot auto-download or download past purchases with a different Apple ID for 90 days" when switching iTunes library from old PC to new PC
It looks like you have been just caught up in a mess with Apple Servers.

Presently there is a stampede going on at Apple Store, with people wanting to preorder the iPone6. 

Reportedly the server was overloaded and had to be taken down, and has not been operating correctly since.

Others have reported similar problems.

Just hang in there till the dust settles, and keep trying.