Uhm,
I’m not sure about the error – 500 error is an internal server error, so it doesn’t help much if there’s an error in code triggering that or is a problem on your server/installation
one possibility is perhaps you had both the old and the new plugin active at the same time? the new one should overwrite the previous one in the same directory (of course the day it’s available on www.remarpro.com there’s no need to move files manually)
the new version also requires php 5.3+, but if the update scripts have run, you probably have that – my guess is that perhaps the script timed out (insufficient server resources? php/webserver configuration issues?)
if you have a backup of the database (a mysql dump or an image of your installation if your host provides that), all your settings and posts are still there, just overwrite the database with the previous one and you’d be done – the plugin settings (2.x and 3.x) are all stored using the WordPress settings API in the ‘wp_options’ table. In 2.x they were prefixed with gce_
in 3.x they are prefixed with simple-calendar_
Hope you can sort it out