There is a bug in the latest v1.4 update which is causing Ultimate Visual Composer Addons to fail to install. The problem is caused by the plugin file having the wrong name. The file can be found in /woopress/framework/plugins/.
Find the file:
Ultimate_VC_Addons 3.9.4.zip
And rename it to:
Ultimate_VC_Addons.zip
Now it will work correctly when installing required theme plugins.