Update: It seems this has something to do with the hotkeys used by Ubuntu/fcitx for switching between keyboard layouts. I had a keyboard-switching hotkey involving a Shift combination, and as long as that was active, the error occurred. Now I've changed the hotkey to something else and the error seems to be gone.
Update: It seems this has something to do with the hotkeys used by Ubuntu/fcitx for switching between keyboard layouts. I had a keyboard-switching hotkey involving a Shift combination, and as long as that was active, the error occurred. Now I've changed the hotkey to something else and the error seems to be gone.