Our VM that runs our jenkins master has the following disks configured and mounted:

Filesystem            Size  Used Avail Use% Mounted on
/dev/mapper/system-root
                       12G   11G  901M  93% /
tmpfs                 939M     0  939M   0% /dev/shm
/dev/sda1             124M   32M   87M  27% /boot
/dev/mapper/system-log
                      3.0G  104M  2.8G   4% /var/log
/dev/sdb               20G  5.5G   14G  29% /var/lib/jenkins

As you can see, the space on "/" is rather low. Jenkins therefor disables the master Build Slave.
This should not happen, since JENKINS_HOME is set to /var/lib/jenkins and java.io.tmpdir is set to /var/lib/jenkins/tmp. The disk those directories are on still have plenty of space. So there is no reason for jenkins to disable the master node.