Some days ago I upgraded a vSphere 4.0 customer infrastructure to vSphere 5.1.
Everything went smoothly. After a few days, the customer called me complaining that he could no longer do the copy paste from her pc.
Starting with vSphere 4.1, the Copy and Paste options are, by default, disabled due to security concerns.
I remember that it was possible to enable this option for a specific virtual machine.
After reading the KB I found that now is also possible to enable this option for all the virtual machines in the ESX/ESXi host.
To do this you need to SSH access to the host and add these entries to the file /etc/vmware/config
vmx.fullpath = "/bin/vmx"
Shutdown and power-on the virtual machine for changes to take effect.
This is the link to the VMware KB http://kb.vmware.com/kb/1026437