This is a tough one because this problem is usually transient and self-corrects on the next run, however, if it fails for multiple runs, action will almost certainly be required from the end user to correct the issue. I just had this exact situation come up twice in the last week, both customers were seeing very long backup times, running into their production day and causing issues (in one case aborting due to backup window), but there's "nothing wrong" according to Veeam. Investigating turned up mulitple VMs with the invalid CBT issue every night but the customer scanned right over the message in the event log because it was green. Resetting CBT for the impacted VMs corrected the issue after one additional run.
It would be really nice if we could potentially turn this into a warning only in cases where the previous sessions had also experienced the failure. Perhaps another option would be to use a blue "info" status for such a message (I know, we don't really have that status today, but I could see it's use in other cases as well), something that doesn't change the overall status of the job/task, but would stand out when scanning the task log. Just throwing out some ideas, I certainly understand our current reasoning, but it's somewhat misleading when this failure happens to be persistent rather than transient.