/ul - This was an old command by Minehut before they "Unleashed"
An error occurred, please try again. - 9 times out of 10 the file is just uploading. Don't do anything on the page and wait a couple minutes.
World panel doesn't work - I've never used the world panel to upload worlds, since you can upload the .zip file to the 'home' file and unzip it there. This could be the issue as well.