I had an issue when using update manager to scan a host and it would fail with a “Check log for errors blah blah blah”. The errors are never useful. After investigating and checking the log, I found the following entry:
2015-06-02T20:57:30.312-07:00 [00504 info ‘VcIntegrity’] Error on logout (ignored): vim.fault.NotAuthenticated
When researching the issue, I came across a best practices guide that basically pointed out the error. So as a general best practice, verify on your hosts, on the configuration tab, under DNS and Routing, that you have filled in all the fields. I had missed the Search Domain field on that particular host which was causing the scan to fail.
As always, leave questions or suggestions in the comments.