We have exactly the same problem here. We are a french school of Design, equipped with a very fast network (gigabit local, and 100 Mbit fiber internet line).
Impossible to download anything using Adobe Creative Cloud Packager. Error A12E5. We have tried everything tip we found on the internet : Reset or delete OOBE files, uninstalling everything and starting from scratch, Adove Cleaner tool (that don't find anything to uninstall, btw) etc.
Our network is working like a charm and don't have a single problem of stability... We only have a proxy server set in OSX preferences that has always worked well with CS ( that we usually downloaded from Adobe Licensing website ) installed using Adobe AAMEE.
With CC that's another story : When starting Creative Cloud Packager, we are unable to even start the process. Error A12E5 comes instantly. And we are unable to get rid of the proxy since we are a School with more than 1000 users... We can't unplug the Proxy-firewall just to make a try... Proxy server can't be the problem anyway since it's correctly set in OSX preferences...
So I am creating our install package at home ( ! ) : Tons and tons of Gigabytes to download on a 4mbit line.
And we fear the next update process that will surely fail with error A12E5... !!!!!!