The 'Native Posix Thread Library' (NPTL) is not supported by User Mode Linux (UML). NPTL is the much-vaunted 'new' threading implementation in the Linux 2.6.x kernels. There can be many symptoms of this problem, but the one I seem to always get is a crash whenever a program tries to resolve a host name. Whatever the symptoms, you will only see problems with a 2.6.x UML kernel - the same filesystem will work correctly when you boot it with a 2.4.x UML kernel.

The solution: Disable the new threading by renaming /lib/tls to /lib/tls.DISABLED on the guest, and reboot UML.

Although this solution is simple, it's not permanent (at least on Debian). apt will continue to re-create the /lib/tls directory whenever it is updated. Then you'll start to see crashes again when you next reboot.