It turns out the problem was in the directory I was using for syncing, that is, /mnt/backup/borg. I reckon the problem was caused by resizing the disk partition where /mnt/backup lies, as I had shrunk that partition before installing OpenMandriva. Though I checked that partition after shrinking it (and before installing OpenMandriva) by creating and deleting files in it, for some reason the command aws s3 sync wouldn’t work specifically with the directory borg. I realized that was the problem as I created a directory /mnt/backup/test and the command worked with this one.
So, I deleted /mnt/backup/borg and created it again. After that, the command ran successfully with that directory, for both root and non-root users. Who knows what happened to the original directory. Weird, but I’m glad it’s solved.