Question:
Unable to connect to MKS: internal error when opening the window, we managed to log in to the vCenter Server using the VSphere Client to operate the virtual machine on the host. However, when logging in to the ESXi host using the VSphere Client, we were Unable to open the window of the virtual machine.

Problem Solving:
1. Check whether port 902 on the Client host is occupied, and release if occupied.
    
The port is not occupied
2. Turn on the SSH function of ESXi host and log in to ESXi.
3. Execute service mgmt-vmware restart
Note: Some people may execute this command after the service may appear is not recognized command, some people may not find this MGMT-VMware service, this time can try the following method.
4. Vi/etc/vmwaer/config
Add a
Vmauthd. Server. AlwaysProxy = “TRUE”
Test whether the service is available, do not restart the ESXi host, and then try again.

All the above methods are not good, then reinstall the vCenter Server! Good luck.