It kinda looks like your error log is saying the 500.shtml page does not exist. When you experience the problem on your live site are you taken to this 500.shtml page? Is there any error message being displayed on the page?
If you are getting redirected to your 500.shtml this would indicate that a server error has occurred. If you do not administer this part of your server you should speak with your host about temporarily disabling the custom error page and print the errors to the screen.
If you are getting the error when clicking on the browse button it may be a permissions issue trying to create a folder in a location that it does not exist. If we can get the specific error message that is being returned by the page we can begin trying to asses the issue.