I see a lot of DB fix entries in the log from the 27th. Do you still get these errors when you restart the SIM service ?
So if you look on the left of your SIm screen, look at events > Shared > Events by Security > All Events you don't see anything ? There should at least be login and logout events. Also look under Options > Events > Event Filter Settings.
The second check, Critical, Major, Minor, Warning & Informational should be on. Do you use SNMP and WBEM for your event trapping ??