I found the thread that appears to be mentioned in your screenshot above; the mark up of your users can be slightly different but I think that is username styling. Even so, the Structured Data tool doesn't flag up any errors. I might need to look into exactly what the spec is here to be sure.
I also had this and found out it where guest accounts from our old vb forum that you could not click (not associated with any group). Strange enough a few registered accounts also. You can check this in the thread with the errors.