I'd really need to see the gallery you're trying to open in order to determine the cause of the problem.
I expect there's a problem with the gallery's files or structure as I don't think there should be a problem otherwise.
Try opening the gallery's 'config.xml' file directly in a browser. If there is a problem with the XML syntax, the browser should let you know what the problem is at at what line the problem occurs.
Manually adding images (as you have done) will not keep their original captions unless the captions are embedded within the images themselves as IPTC data and you select 'Images -> Titles -> IPTC Title' or 'Images -> Captions -> IPTC Caption' from the drop-down menu at the top.
Could you maybe zip one of the galleries that you are trying to open, upload the zip file somewhere (e.g. your own website or a file sharing service such as Dropbox or Google Drive) and send me a link (either here in the forum or via steven@juicebox.net)?
I'll try to open the gallery myself (on a Windows PC, I don't have a Mac) and at least let you know if I see any problem with the gallery.