I was experiencing the same until I rebooted the vCSA after which I managed to upload the ISO. Was looking all over the place for a fix but rebooting the appliance was not on my thing to try list! 'The IT crowd' comes to mind.
[2017-04-25 09:22:00:361 'managedJetty' 140149099587328 INFO] [managedJetty, 368] Jetty: 2017-04-25 09:22:00.360:WARN:oejs.ServletHandler:qtp1066376662-9: Error for /vum-fileupload/login
[2017-04-25 09:22:00:362 'managedJetty' 140149099587328 INFO] [managedJetty, 368] Jetty: java.lang.NoClassDefFoundError: com/vmware/updatemanager/fileupload/LoginWorker$LoginWorkerException
[2017-04-25 09:22:00:362 'managedJetty' 140149100386048 INFO] [managedJetty, 368] Jetty: at java.lang.Class.getDeclaredConstructors0(Native Method)
