I tried autostarting Sunbird via alltray by adding "alltray sunbird" to the list of autostarted apps. This failed, so I wrote a script to start it. This failed also. Finally I added a sleep in my script before calling alltray and it worked. Other apps work fine. My assumption is that because alltray is so simple, it starts quickly and attempts to create a tray icon before the tray is ready for it.
Hi. This does indeed sound like a problem with alltray when there is no running system tray. Unfortunately, there is no way for Xfce to solve this. Alltray could try and wait for a system tray to show up, but that is up to the alltray developers. The systray specification says this: "An application wishing to provide an icon to the system tray should first locate the system tray by requesting the owner window of the manager selection. If the manager selection has no owner, clients may use the method described in the ICCCM (watching for a MANAGER client message) to be notified when a system tray appears." Thank you for the report.