500 Internal Error is from hosting. The Demo file with all images is very large and some hostings may limit import speed/size.
Ideal solution would be to ask hosting to increase your speed/size. If you’re paying a small fee, that will probably not happen.
Alternative is to try doing the import several times and check Media folder to see if images are uploading. Sometimes import takes place, just in background.
If you can’t find a way, I’ll try to help. Send me wp-admin credentials at [email protected] I’m traveling until Jan 17-18, but I’ll try to help as soon as I’m back online.