When reviewing ETL logs, the following kind of error appears with the following key error: there were problems obtaining performance data out of [x] hosts over [y] available...

A sample from the ETL log may appear such as the following:

2017-01-11 12:00:00,002 WARN  [service-1-1]- BCO_ETL_WARN306: From [2017-01-11 1:00:00] to [2017-01-11 2:00:00] there were problems obtaining performance data out of [2] hosts over [10] available in [vmwarehostname] datacenter:[hostnames with errors].

The warning above indicates that data could not be retrieved from 2 hosts.  Looking further in the logs, you can see that that the hosts are being skipped and for what reason:

Additional errors in the log match the number that has been skipped.
2017-01-11 12:00:00,000 INFO  [service-1-1]- Collector: collecting host/vm performance information
2017-01-11 12:00:00,111 INFO  [service-1-1]- [queryVCData]: Skipping perf extraction for DISCONNECTED host: [hostname with error 1]
2017-01-11 12:00:00,111 INFO  [service-1-1]- [queryVCData]: Skipping perf extraction for DISCONNECTED host: [hostname with error 2]

The error showing the skip indicates that the DISCONNECTED host is no longer available. This is an error on the VMware side, not TSCO.  

VMware Admins on the customer side should verify the availability of these hosts in order to collect data against them.