The situation has evolved.
I cannot open apps manager (apt-get or dpkg fails because of disk full)
I cannot either open the preview of my pictures, only see one by one from file manager.

df shows a roots partition with 0 space available. (sometimes, df does not even work)

So it looks like my (too small) root partition is full.

Since dpkg does not work, what can I safely remove and how to make a little space and remove some apps correctly afer ?