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Afer about 66 seconds, Dropbox closed the connection on our face. Go to Administer Backup Files, find the latest backup attempt (the topmost row) and click on the Transfer archive link. There's a good chance this will transfer the file successfully.

Most likely this issue was just one of those infrequent cases where a network issue beyond yours and our control prevented the transfer from working. I would not recommend changing any setting. For what is worth, I get 4-6 failed transfers to S3 every year. My backup settings and host have not been changed for the last two years.

Go to Akeeba Backup and make sure the Dropbox-enabled backup profile is currently selected. If not, select it from the drop-down and wait for the page to reload.

Now click on Configuration. Find the Archiver Engine row and click on the Configure button next to it. A pane opens below. Set the part size of split archives to 20Mb or even 10Mb (lower is more stable). Take a new backup.

The backup now transfers to Dropbox as multiple files (.jpa, .j01, ...). You just need to have all of those files present when restoring a backup. The idea is that the smaller files you have, the faster they transfer. The faster they transfer, the less time it takes to transfer each file. The less time it takes to transfer each file the less is the possibility of a timeout or a network issue interrupting the file upload. Actually, this is the advice I do give in the documentaiton, where I also explain the thinking behind this in more detail.

Give it a day and retry the backup. I am pretty sure this is a temporary issue but the only way to know for sure is, well, retrying later.