losing sessions for some users?

Hi, I'm working on setting up an XWiki-based site for a small community of users. All has gone well for the most part, but I've got one issue that I'm hoping someone out there can shed some light on. I have about 60 users that are members of a group that has read/write access to the Wiki, and logging in, editing, etc. works just fine for the majority of users, but for a small subset (3 users I'm aware of) after login, they get redirected to the main page as they should, but it appears the site is unaware of who they are. For instance, where it normally would say "Hello Joe", it says "Hello $docuser.display("first_name", "view", $objuser)". They also get the message "You are not allowed to view this document or perform this action" when viewing pages they should be able to access. The problem doesn't seem to be related to the actual users themselves, because I had one of the users login with someone elses computer, and it was successful. But for all 3 users, switching between IE and Firefox doesn't help, and I haven't been able to find any browser settings that help either. I have also been unable to reconfigure my own browser to reproduce the problem. Does anyone have any ideas about what might be causing this? My best guess is that somehow the browser is not presenting a session ID that matches the user's actual session object, but I don't know why that might happen. I'm relatively new to both WXiki and Tomcat, so I'm not sure what logging options might help me see some relevant information. I see many messages like this in my logs, but they occur even when users that do not exhibit this problem hit the site, so I'm not sure they are related. 14:39:10,729 WARN TP-Processor1 http://<site>/xwiki/bin/view/OpsTeam.Record/Schedule XWikiStatsServiceImpl:findVisit:314 - Found visit with cookie 8YXUJFTIQSJJA1CQRCDJ0RXGMZAVS8XY in session 2E90CB72684FB68726A51AFA5E6B6D13 for request with cookie 5QZRQ6GC89SGSSBCZPTQOJQXJ9WT2BQG I'm using XWiki 0.9.840 on Tomcat 5.5.16 behind apache/mod_jk Thanks for any help or ideas you can provide, Kyle