Hello we experience rare but constant problems with "perfect server" on Ubuntu 8.04 + ISPconfig 2 (different ispconfig releases and also the last one).
The main play is around the /etc/shadow (contains the linux users passwords in encrypted format, http://en.wikipedia.org/wiki/Shadow_password). The problem is that when we modify the datas for one user (ex. "web25_info") then the /etc/shadow will rewritten for all domain users "web25" even when no password was modified!!!
So the MAIN problem is that sometimes (could not reproduce it in vitro) the password changes to some uncertain one, so that any authentication versus /etc/shadow for the user under modification FAILS.
The problem causes authentication error with ftp, ssh, pop and other services.
The unique solution we found is to backup /etc/shadow, introduce any modification to the user settings (ex. activation antivirus/antispam for the user email) and then recover correct shadow from backup.
Any ideas or advice on the question are extremely welcome!!