Why do I see the following errors on my FreeBSD server? kvm_nlist failed swapinfo: undefined symbol: _numvnodes

Most likely you have /boot on its own partition. On FreeBSD, /boot should be on the root (/) partition; otherwise, it will not be able to use /boot/loader(8) and force the system to fall back directly on the /kernel.

To fix this, you need to copy the data out of /boot/ to a directory on /, unmount /boot, remove it from /etc/fstab, then move the contents that were in /boot to the (now plain directory) /boot/ . Once you reboot, it should be working normally.