Try running swapoff on this swapfile and then swapon again.
Same issue. It will create the swap file fine but it won't start it. Let me know what info you might need from me to troubleshoot.
root@VOID:~# swapoff -v /mnt/cache/swapfile
swapoff on /mnt/cache/swapfile
swapoff: /mnt/cache/swapfile: swapoff failed: Invalid argument
root@VOID:~# swapon -v /mnt/cache/swapfile
swapon on /mnt/cache/swapfile
swapon: /mnt/cache/swapfile: found swap signature: version 1, page-size 4, same byte order
swapon: /mnt/cache/swapfile: pagesize=4096, swapsize=4294967296, devsize=4294967296
swapon: /mnt/cache/swapfile: swapon failed: Invalid argument