Admittedly, this is a guess.
- The exporter, in the versions I have used, does not save your files
to a local disk. It creates an XML file with references to your
images on a server somewhere. - When the importer reads the XML file, it will download from that
server and upload to your new installation.
If your images are not available– not accessible for download– the importer cannot access the server where the images are stored, you will get upload failures. I am guessing that that is what is happening. Perhaps, the old server is refusing connections, or DNS is preventing access, or the old site has been taken down.
If it is a DNS problem, you should be able to solve it by editing the XML file to replace the domain name with the numeric server IP. Untested, but can’t think of why that wouldn’t work.
If the site has been taken down, I hope you have backups of the images somewhere.