For multi-user setups, UPHClean is a good utility. AV software and hardware drivers can interfere with the logoff process according to Microsoft. User hive issues would not be good and could cause data loss. Hives are another name for part of the registry.
DWLPL, let me ask you this:
Do you get the same logged event when using the Welcome screen and the blank screen issue appears and is it always the same user?
If so, a visit to Control Panel Add/Remove to uninstall this item may help.
Alternatively, an investigation into the cause of the event by process of elimination by closing any TSRs, Anti-Virus and hardware drivers/software that are presented in the tray as an icon (next to the clock) one-by-one on the user(s) desktop prior to logoff, then checking the event viewer. A bit tedious, yes.
Good Luck!