[Milhouse]

Internal error isn't good, do you actually have any Sets defined? I'm guessing you ran the jd option without specifying a media class - try "jd artists" or even include a filter (jd artists beatles) to limit the output produced rather than running jd for pretty much everything.

Enable the webserver in Settings -> Services -> Webserver

Once you have enabled the webserver, use the "c artists" option to pre-load just your artists artwork. Replace c with C and specify a filter - a partial artist name - if you need to "force" items into the cache. Enable allow.recacheall=yes in properties to force the caching of an entire media class (or append @allow.recacheall=yes to the command line).