Looks like you have a problem in the cluster connection and it`s barely related to Veeam One.
However, let`s try to troubleshoot the problem.
Host could not reach the isolation address, means the cluster is not functioning correctly. The default isolation response address is the default gateway but other reliable IP addresses can be configured along with the default gateway or the default gateway can also be disabled as an isolation response address.
I`d suggest to investigate your network to determine why it is unable to receive a response from at least one isolation response address.
If the isolation response address was manually added using the advanced setting das.isolationaddress
remove the non-pingable isolation address from your advanced settings and specify a new pingable isolation address.
When the listed ip address is the currently configured default gateway, add an additional isolation address using das.isolationaddress
and disable the use of the default gateway for isolation verification using the advanced setting das.usedefaultisolationaddress
set to false.
I believe VMware support is the one who solve the problem for sure. Also review this KB
for more info.