I ran into the following error under the plugin section of OpenMediaVault after a clean install.
Error message: openmediavault, Failed to execute command ‘export LANG=C; apt-get ..etc

To resolve this I needed to edit the sources.list with the proper source information below.

# Debian Squeeze Sources
deb http://ftp.de.debian.org/debian/ squeeze main non-free contrib
deb-src http://ftp.de.debian.org/debian/ squeeze main non-free contrib

# Debian Squeeze Updates
deb http://security.debian.org/ squeeze/updates main contrib non-free
deb-src http://security.debian.org/ squeeze/updates main contrib non-free

# squeeze-updates, previously known as ‘volatile’
deb http://ftp.de.debian.org/debian/ squeeze-updates main contrib non-free
deb-src http://ftp.de.debian.org/debian/ squeeze-updates main contrib non-free

Suspected cause:

I did an offline install and skilled the repositories and they were never added