I'm getting the same error after doing an upgrade from 3.8.7-1 tot 3.8.8-1:
A a user who had a few reminders set before the upgrade is now getting the error "Can't call method "id" on an undefined value at /usr/share/request-tracker3.8/html/Elements/MyReminders line 58." on the Startpage. I tried setting the LogStackTraces setting to 'error' as posted above, but even after an apache restart I'm not getting any information logged. Disabling the Reminders box on the Startpage prevents the error from happening. It also works fine if I make a new ticket, add a reminder and go back to the Startpage. It just seems to go wrong for tickets that had reminders set before the upgrade from 3.8.7-1 to 3.8.8-1 Hopes this helps.
