Ah. Not letting a free space erase complete makes it pretty much useless, because it's impossible to say (without checking, for example with a file recovery program) whether all the sensitive data has been overwritten or not. Also, the MFT is not cleared until after the free space erase has been completed, so there could be entries relating to sensitive data. Sadly a free space erase will always run for a number of hours, and much longer than that if anything other than single pass erasing (only one pass is needed) is used, or the machine is slow and/or the drive is large. Typically, hard drive performance is the limiting factor.
Particularly on drive C:, there will always be error messages (lots of them, in the log) on a free space erase, particularly on Drive C: if erasing of cluster tips is enabled. Most of these messages are quite unavoidable, because they relate to Windows protected files, and they are therefore, in my opinion, spurious. This is not a bug, because the behaviour is designed, but it is (again in my opinon) a fault, as the user does not need thousands of messages about which she or he can do absolutely nothing. Also, the messages reside in the task list file, taking up space and probably memory when Eraser is running, which makes it a good idea to empty the log for free space erasing tasks.
Going back to the tray icon, Windows seems all to eager to make system tray icons invisible, but they may still be present. If you click on the tiny upwards pointing triangle on the left of the tray, a small window appears with icons that are not visible in the tray. The Eraser icon (which is a miniature version of the desktop icon) may appear here. If it doesn't, click on the 'Customize' link in the window, and you will see options for each icon; for Eraser you will need 'show icon and notifications' to be set. And if the icon is still invisible, open the Eraser program from the desktop and the Start Menu, and the icon should appear.