Well, I found out yesterday why those downloaded files wouldn't work - they're corrupted.
I didn't figure that out until I tried to do them again and noticed an http download alternative. I tried that and all ok. So then I investigated a bit. I examined the originals with WinRAR and it reported that they all were corrupted in various ways. They were all ftp downloads via my browser (Slimbrowser).
I tried ftp downloads again, this time with an ftp client (Cute FTP). No problems. I tried other sites that offer ftp downloads. Through the browser, all were corrupted. Through CuteFTP or http, if offered, all were ok.
Soooo.... now the question is, why would the browser reliably corrupt ftp downloads, but not http? (in the browser setup I don't use a proxy for ftp downloads).
I don't need and answer, but would be interested if you have a suggestion.
AA