I just entered this bug entry. It cropped up when I went to install the VirtualBox extension for USB support. It kept complaining about lacking the proper entry in /etc/shells, so I took a look and found that there were two entries for /bin/dash and none for /bin/bash. Once I changed one of those entries to /bin/bash, things started working properly again.
It is interesting that I can log on with that entry missing, but it definitely shows up when I use a command that requires /bin/bash.
This is the /etc/shells file from my desktop machine. as it was before I
This is the file after I modified it.
How many others have this? Is it a problem with preupgrade or does a fresh install also have this problem?