I wish I could offer a solution. I have had this problem for months, specifically for birthdays: I would get reminders that I could not snooze or dismiss. I find lots of reports of other people having the same issue, but no "magic bullets". Strangely, the problem seemed to have gone away recently until a couple of weeks ago when I got another one, but none since. I've had to stop using Thunderbird's calendar. I still hope to learn the cause and fix - I could really use those reminders.