I found, after upgrading Veeam One to version 9.5, that a host to be monitored was selected by the automatic exclusion rule.
This is the new product behavior to avoid license conflicts, in the previous version there was a specific “Monitored Host” tab where to specify exclusions.
In this case, node esx02 was selected instead of the previously, esx03.
To exclude the correct node, a new rule has to be created.
Select to exclude VM’s monitoring by infrastructure location.
Click Finish to create the rule.
After a while the correct host is monitored again.
The new way to manage host exclusion is well explained in the user guides as well by the specific KB article.