I had the same problem plus some others. The clear cache command fixed the others but not this one. I later discovered that the "Who's Online Timeframe" value was blank. I believe in 6.3 you couldn't set this value so maybe that's why it's blank after a 6.3 to 6.5 upgrade. Anyway, setting this parameter to 10 minutes resolved the problem. This also explains why I seemed to be the only one ever listed on the Who's Online display.
You'll find the Timeframe parameter under Display Options in the General options section on the Control panel.
