Relevant to those with STANDARD installation donwloaded from Authors site:
The reason why there is a fatal error is occurring if i remember correctly, the version on wordpress.org differs from the "standard" version downloadable from the plugins author site. SO, if you use the STANDARD version on your site DO NOT UPDATE using the reminder within your wordpress installation ie its fed from wordpress.org! - as it will think you are using the lite version and will attempt to update your standard version (as you have installed this) and ultimately break the plugin!
Ok here is a WORK AROUND IF YOU HAVE ENCOUNTERED THE FATAL ERROR which worked for me:
1/ Firstly you need to access via FTP such as dreamweaver etc the directory containing the plugin ie wp-content/plugins/all-in-one-event-calendar
Using the FTP change the name of the folder from "all-in-one-event-calendar" to anything you want in my case i simply used "123". By doing this you force WP to deactivate the plugin on the plugin admin area. This is particularly helpful again such in my case, where by i couldnt even access the plugins list in admin due to the fatal error occurring!
2/ Now check the plugins list via your website and it should show that the plugin has been de-activated.
3/ Download the STANDARD version of the plugin from http://time.ly/ (v 1.10.11).
4/ decompress the file - you should have all-in-one-event-calendar folder with all the contents
5/ Upload the the entirety of the all-in-one-event-calendar folder to the plugins folder in your wordpress installation USING YOUR FTP.
6/ Once the upload is completed go back to your wordpress website plugins admin page and activate the plugin - error should no longer occur.
Hope this helps