We are using 6.0.2 GA Network Edition 64bit on Ubuntu 8.04 lts. Having a strange problem with a user's account. Makes me think there is something in the account that is causing this behavior. It doesn't seem to be browser related because we tried it on Firefox 3.x (different platforms, OS X, Linux and Windows), IE 8 and get same results.
This is what happens. Open 'preferences', highlight 'mail' in the left frame navigation menu, 'Receiving Messages'>'Send auto-reply message:' is displayed in the main frame. The 'Start on' and 'End on' for other accounts has a little pop-up calendar. This particular account doesn't show that. Also if you try to save preferences without making any changes, it gives the error message 'A required value is either missing or invalid'. There is no way to modify preferences in this account.