Hmm. That's a curious one. The error is to do with the comparison check the plugin runs if display of both Steam and forum names is enabled. That error indicates that for a user on your forums, he has a Steam ID associated but no Steam name, or for some reason his forum name wasn't saved to the cache, since it isn't finding one or the other.
I can tell you that you definitely won't see it again if you set the display name to just display forum or Steam name, but obviously the setting for both is supposed to work as well (I use it myself on my site). Do you get this error routinely, or did you only get it once? Are there any anomalies on the status list (missing name, etc.)? If you could check Admin CP > Tools & Maintenance > Cache Manager > mysteam, and then look at the cache to see if you find any users with a blank 'username' or 'steamname', that would help to pinpoint it as well.
I can tell you that you definitely won't see it again if you set the display name to just display forum or Steam name, but obviously the setting for both is supposed to work as well (I use it myself on my site). Do you get this error routinely, or did you only get it once? Are there any anomalies on the status list (missing name, etc.)? If you could check Admin CP > Tools & Maintenance > Cache Manager > mysteam, and then look at the cache to see if you find any users with a blank 'username' or 'steamname', that would help to pinpoint it as well.