I've found that performing the forum maintenance options located in admin at Badman's (also SMF) helps when people sometimes receive database errors. Especially optimizing the tables to improve performance and recounting all forum totals and statistics. You may also want to check the error log (if you don't have it enabled the option is in features and settings). It can sometimes pinpoint where and what a problem is.
Thanks JP - I try to do the forum maintenance thing every couple of days and just did it again to see if it would help. It didn't fix the error. I believe it has to do with some of the mods that I've installed with the forum that may be interfering with the viewing of who's online.
I'm going to contact some of the mod authors to see if there is a quick fix for this. I'll let ya know if I can get it fixed.
I have had a few times when I have "hidden" my online status for one reason or another and so the little computer screen is not always an indicator of the online status of a person if they do not want to be "seen". Could this be one of the problems?