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When the other person is using the account to listen, one of my go-to tricks is to enter a dummy IP address (usually 127.0.0.1 with a random port) to fake "airplane mode" - Spotify can't connect to the internet and detect the other's usage, and I can listen to downloaded playlists. Win-win.
But! Recently I noticed that filling out a dummy proxy IP will simply cause Spotify to show a "Something went wrong" modal, with an unhelpful "Try reloading this page" and "reload page" button, while disabling any access to other pages (including "Settings" page), so I can't revert the proxy settings. Only thing to do is to completely uninstall and reinstall.
Just thought I'd share. Cheers!