Update Manager Error probably due to Squeeze sources

Hi there,
since the end of the LTS-support of Debian Squeeze (29. February 2016) I cant update my Stoneburner installation anymore via Update Manager --> error.
In the terminal I get with apt-get update:

Err http://http.us.debian.org squeeze/main Sources 
404 Not Found [IP: 128.61.240.89 80]
Err http://http.us.debian.org squeeze/contrib Sources 
404 Not Found [IP: 128.61.240.89 80]
Err http://http.us.debian.org squeeze/non-free Sources 
404 Not Found [IP: 128.61.240.89 80]
Err http://http.us.debian.org squeeze/main amd64 Packages 
404 Not Found [IP: 128.61.240.89 80]
Err http://http.us.debian.org squeeze/contrib amd64 Packages 
404 Not Found [IP: 128.61.240.89 80]
Err http://http.us.debian.org squeeze/non-free amd64 Packages 
404 Not Found [IP: 128.61.240.89 80]

Searching around in /etc/apt/sources.list.d/ I found one source which could be the problem:
cat openmediavault-millers.list

#######Millers - Sources list#######
deb http://http.us.debian.org/debian/ squeeze main contrib non-free
deb-src http://http.us.debian.org/debian/ squeeze main contrib non-free
deb http://debian.linuxmint.com/latest/multimedia testing main non-free

So I do not know for what the source list is for, I suppose it is for a plugin I once installedI would like to ask whether I could erase openmediavault-millers.list and get rid of the update problem?

Thanks for a hint in advance,
topi

Thanks ryecoaaron.
At the end it was a leftover from an installation of the Sickbeard+SABnzbd+Couchpotato+Headphones+Others script [GUIDE] Sickbeard+SABnzbd+Couchpotato+Headphones+Others I did in the days of OMV 0.5.x .. jhmiller advised me to safely delete openmediavault-millers.list and afterwards to apt-get update. So now my NAS updates again without any issues