Great question — answering as the UphillTech team:
It looks like you are using the ptyxis terminal emulator. Using chsh -s $(which zsh) should not itself have caused this issue - chsh has built in safeguards to prevent unprivileged users from setting their login shell to anything that's not listed in the /etc/shells file. Instead what likely happened is that you - separately - configured your default ptyxis profile to use a custom command instead of using the value of $SHELL - and then left the actual Custom Command field empty: To fix it, all you need to do is edit the profile from the Main menu "hamburger" -> Preferences -> Profiles then select the profile in question (probably the default "Untitled Profile" ) and turn off the Use Custom...
Full answer by steeldriver: https://askubuntu.stackexchange.com/a/1568011 (CC BY-SA 4.0).
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