This is a known issue and the workaround is to use a older version of browser of Mozilla Firefox.
From the release notes: https://www.vmware.com/support/vsphere5/doc/vsphere-vcenter-server-55u1-release-notes.html#knownissues
Client Integration Plug-In might not work as expected if you log in to the vSphere Web Client using Internet Explorer 11*
If you log in to the vSphere Web Client using Internet Explorer 11, the following problems might occur when you perform tasks related to Client Integration Plug-In:
- Even if the Client Integration Plug-In is installed, the following message is displayed when you attempt to import or export an OVF template:
Download the Client Integration Plug-in - File browser does not allow you to upload or download a file from the datastore.
- You cannot mount images from the CD/DVD drive.
- The virtual machine console might display a black screen with the following error message:
Remote Console is not properly installed
Workaround: Use Mozilla Firefox, Google Chrome, or earlier versions of Internet Explorer to access the vSphere Web Client. You can also use vSphere Client instead of vSphere Web Client to work around this issue.