According to http://blog.sanctum.geek.nz/term-strings/:
" A certain piece of very misleading advice is often given online to users having problems with the way certain command-line applications are displaying in their terminals. [...] This misinformation sometimes extends to suggesting that users put the forced TERM change into their shell startup scripts. The reason this is such a bad idea is that it forces your shell to assume what your terminal is, and thereby disregards the initial terminal identity string sent by the emulator."
Following their suggestion, I have erased from ~/.bashrc, ~/.vimrc and ~/.tmux.conf every mention to the terminal.
Now I have at Konsole:
user:~$ echo $TERM xterm
at tmux:
user:~$ echo $TERM xterm-256color
and everything works well.