Have reviewed your install. The upload is actually going through fine. You can see the media entry in backend which previews fine. Uploading from backend also works flawlessly.
It is undoubtedly a 3rd party extension causing this. I've edited your Joomla templates error page temporarily to echo out the stack trace and found the following.
Code:
JROOT/plugins/system/GoogleAnalytics/GoogleAnalytics.php:64
Your issue is caused by 3rd party extension "System - Google Analytics". Unpublishing it resolves your issue, which I have done for you. Suggest finding a different Google Analytics plugin or letting the developer know of this issue.