Are you updating the license from the GUI, hitting the 'Activate Firewall' button from there? Or are you generating a license HTML file from the activations site? I have never seen this error, nor do I find it in our engineering escalations. It sounds to me like one of the values you have entered yourself in the License screen has a \ in it somewhere.
tried both the GUI, hitting activate, and using the cf lic get in the CLI.
(Yes, that's four backslashes, which searches for one \ when interpreted by the shell)
$> cf -J lic q | grep \\\\