that's not an error brah -- that's just saying leave the window open and after a bit it will pick up where it left off
because what's happening when you click 'mass download' with 500+ audio files selected, each about 6MB or more, is a script is running on the server that is zipping up all of those files -- and the makers of this forum software (SMF) realized that most people who host websites had contracts with webspace companies that would terminate massive downloads because they take too long and use up too many resources in the short term. so they built it into the mass download script that it would take a break for a few seconds -- in order to make sure the entire contents you wanted got zipped up into one file for you to download, but without using too many of the webspace company's server resources
so just wait a minute. the script will keep going and eventually the entire list will be compiled into one giant zip file (probably close to 3GB...) for you to download...
but in reality, it would take probably less time to right-click each one and click 'save as'...
on the bright side, you only have to do it once, if you back them up in multiple places.