[PROBLEM] Updates not working

[syco]

I realised some time ago and again today, that updates will not work on my Xbian installation.

That is the error code, when I try to update:

E: Could not get lock /var/lib/dpkg/lock - open (11: Resource temporarily unavailable)
E: Unable to lock the administration directory (/var/lib/dpkg/), is another process using it?

Before the update I rebooted the Pi.

I have totally no clue, what is going on. I have a very fresh installation of Xbian (one month or so) with absolutely no changes/alteration of the system. I even did not install plugins, as far as I know.

Anybody knows how to solve this?

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Okay, after another reboot I tried this, which worked:

sudo su
password entry
apt-get update && apt-get upgrade

My question is, why does not work the official way (you know, the blue interface with putty)?

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"The packages versions you are trying to install could not be found" appears, when trying to update over the Xbian interface in putty.