How to work with ESXI 6.5 using old vsphere client

sysop

Change tag exactVersion on 6.0.0 in file

/usr/lib/vmware/hostd/docroot/client/clients.xml

Or, more thorough, replace it with a file from ESXi 6.0

<ConfigRoot>
  <clientConnection id="0000">
    <authdPort>902</authdPort>
    <version>6</version>
    <exactVersion>6.0.0</exactVersion>
    <patchVersion>1.0.0</patchVersion>
    <apiVersion>dev</apiVersion>
    <downloadUrl>http://vsphereclient.vmware.com/vsphereclient/VMware-viclient-all-6.0.0.exe</downloadUrl>
    <flexClientVersion>6.0.0</flexClientVersion>
  </clientConnection>
</ConfigRoot>

To do this, create the file /bootbank/clients.xml and enter in
custom boot script /etc/rc.local.d/local.sh

# mv  /usr/lib/vmware/hostd/docroot/client/clients.xml 
      /usr/lib/vmware/hostd/docroot/client/clients.xml-orig

[ -f /bootbank/clients.xml ]&&rm /usr/lib/vmware/hostd/docroot/client/clients.xml
cp /bootbank/clients.xml /usr/lib/vmware/hostd/docroot/client/clients.xml

Reboot the ESXI host.