When attempting to add an ESX host to vCenter, you may get one of the following errors:
Cannot install the vCenter agent service: Cannot upload agent.
Cannot install the vCenter agent service: unknown installer error
Below is a list of most common errors:
Check whether the host is accessible through port 902/tcp. Check the firewall settings and/or routing between the ESX hosts and the vCenter server.
Check whether there is sufficient space on the ESX disk. If there's not enough space, this often gets logged in /var/log/syslog.log during the installation of a VIB. Usually the problem occurs with the tmp partition (i.e. scratch disk) being full or almost full. You need about 100MB of free space for the installation of the agent to succeed. If you have less, try trimming log files by clearing them (i.e. not deleting them):
Sometimes, the ESX host will require a reboot (visible in /var/log/fdm-installer.log. Either reboot the host, or put the host into maintenance mode to resolve.