Some days ago, I got an error RPC:S-5:AEC-0 when trying to download or upgrade apps from Google Android Market.
I tried many things (clear cache, data, restart, clear Dalvik cache ...), but nothing has worked.
On Google support page I found, that I'm not alone with this error. But there was no solution!
It is interesting that this issue was first reported on the end of March 2013.
After some hours of reading, I found a solution. I have to delete Google account on my Android phone and then add Google account back (with same username of course). And from that time, everything is fine again.