This is of course displayed in the system info panel of Virtualmin.
I have seen this issue before and it was typically due to old kernels taking up too much space in /boot
Looks like there should be plenty of room, and I have removed all unused kernels to free space.
# df -h
Filesystem Size Used Avail Use% Mounted on
/dev/xvda2 5.0G 3.6G 1.2G 77% /
tmpfs 1.9G 0 1.9G 0% /dev/shm
/dev/xvda1 107M 26M 76M 25% /mnt/boot
/dev/xvdf 50G 711M 47G 2% /var
/dev/xvdj 50G 572M 47G 2% /home