libstdc++6 version 4.9 or newer needed in the repositories
After an update of a program it requires now the libstdc++6 packet in the version 4.9 or higher. But in the trisquel repositories is the highest version 4.8.2.
Which program?
You can get it from a PPA (which is supposed to only contain free software, but you will use it at your own risks):
$ sudo add-apt-repository ppa:ubuntu-toolchain-r/test
$ sudo apt-get update
$ sudo apt-get install libstdc++6
Thanks a lot. That fixed the issue.