And again another weird XenServer issue here, but luckily this one was super simple to solve. After rebooting my XenServer 6.5 and trying to launch a VM, I got this error message in xsconsole:

Failed: The specified host is disabled

It goes without saying that I did not disable the host myself before the reboot. The funny thing is that everything seemed to work as usual (SSH access, local shell etc), the only problem was that I could not start any VM.

I just took a look at the documentation and I saw that there is a command to manually re-enable a host:

xe host-enable

Simply running this command allowed me to start virtual machines on this host again.