VRA tab on ZVM GUI appears blank. You should see that the VRA tab has an accurate counter of VRAs installed. But none of them or their hosts are listed.
ZVM is unable to properly display VRAs and hosts due to an underlying problem with at least one of the hosts. The problems can be related to disconnection of hosts from vCenter, removal from inventory, or any action which might result in a change of host ID from the one appearing in ZVM database.
The recommended course of action would be to upgrade Zerto to 4.0U3, which will restore the availability of VRAs and hosts.
Alternatively, if you are unable to upgrade, you will need to rebuild the environment.
4.0, 4.0U1, 4.0U2