Upgrading from Etch to Lenny
Problem : You've upgraded from Etch to Lenny, guests start just fine but hang when trying to connect to 127.0.0.1 (in my case it was Courier imapd, however FAM and most other things that access portmap will likely also break).
Cause : The Debian vserver kernel in Etch used a 2.0 series patch, while Lenny uses a 2.3 series patch. Between the two some impressive work was done and they way loopback addresses was handled was changed. The Debian Lenny kernel has CONFIG_VSERVER_AUTO_LBACK set, so it maps all guest access to 127.0.0.1 automagically and in a way that JFW (mostly). The file /etc/vservers/$VSERVERNAME/interfaces/lback contains the IP address that it is mapped to (for the given VSERVER).
Solution : To get the same behavior as the default Etch kernel, you will need to put the 'public' IP of the vserver into lback (i.e. cat /etc/vservers/$VSERVERNAME/interfaces/0/ip > /etc/vservers/$VSERVERNAME/interfaces/lback ). Then restart the vserver and everything should work as expected.
See Installation_on_Debian for other issues.