I too had this problem. The little red triangle with exclamation mark would appear by the update section indicating an error had occurred. However the tray icon remained green, so unless I checked I would never had known.
This only happened if there was an update available, and from a cold boot. My system is Windows 8 and my system disk is a 125MB SSD.
Got a feeling this is something to do with the fast startup facility in windows 8 along with the speed of the SSD
I only installed the trial version, so I was pleased I hadnt paid....
Is anybody from ESET looking into this... I would have purchased but for this....