KB ID 0001664
I had this problem (on sphere 6.7) the other day when trying to deploy some OVA files on my test network.
Well as stated elsewhere I tried reconnecting to my vCenter using its FQDN, this didn’t solve the problem, using Flash or HTML5 didn’t cure the problem either. What did cure the problem was using a different browser! I switched from IE to Chrome and it worked fine.
Update: I Also cured this problem by using Microsoft Edge (The new chromium based one).