When adding the 2 day appointment - are you doing this from a client, the web interface, or your phone? Have you tried this same process but after disconnecting all mail clients including your phone from the account? I've seen android do some weird stuff with appointments in the past!
Just a random idea here. Since your accounts have been around (and using) SM 16 since the early days, it may not be a bad idea to trash your existing calendar XML files and let the later minor version of 16 recreate them. If you have any bad formatting or corrupt entries left over in there - they could be causing some issues.
Kyle Kerst Cameron Solutions LLC www.cameron-solutions.com