Reinstall resolved the issue.
I know it's really late but I just encountered this in a training lab.
In this environment we'd just installed AutoDeploy (no vCenter services had restarted since the install). The solution for me was to restart vCenter. If I'd thought about it a little more I'd have tried restarting vCenter Management WebServices first, but I'll leave that to the next person who comes across this issue.
In my case the problem was that the cache was full. Standard size is 1 GB and I changed it to 3 GB and now the upload is successfully.