@gvimlan yes, the
export command only sets the environment-variable
LC_CTYPE for that session. it's just for testing whether I've determined the correct problem.
So you now know the problem is definitely a wrong assigned environment-variable.
Because I don't use ubuntu at all I don't know where they typically set such variables.
You may also check the /etc/environment file for
cat /etc/environment | grep LC_CTYPE for example) or ~/.bashrc or any other file starting with ~/.bash.
Otherwise it doesn't look like a standard-file for environment-setup does this assignment.
If you don't find that assignment as a not-the-cleanest fix you should overwrite it yourself by adding
LC_CTYPE=en_US.UTF-8 into /etc/default/locale or /etc/locale.conf or (user-scope)
export LC_CTYPE=en_US.UTF-8 into ~/.profile.
For further information about where/when/whatever of environment-variables within ubuntu take a look at https://help.ubuntu.com/community/EnvironmentVariables