Imunify360 blocks ipv6 /64 networks.
In ipv6, the first 64 bits define a node (basically a device: desktop, cellphone, VM) while the second 64 bits are used for routing inside the node.
You can try to add 2001:41d0:52:300::/64 instead.
We'll improve UI validation messages and will probably add some logic to handle this case better (e.g. accept /128, but block the whole /64 and display a message).