I have found out a solution by myself: iBus wasn't working and I was closing it from the tray unknowingly. I have edited my .xinputrc to allow iBus to start. Still, I don't know why on the other accounts it worked normally; I will mention the contents of the edited .xinputrc below.
# im-config(8) generated on Sun, 07 Feb 2016 00:01:26 +0200
run_im ibus
XIM=none
XIM_PROGRAM=
XIM_ARGS=
GTK_IM_MODULE=gtk-im-module-simple
QT_IM_MODULE=xim
IMSETTINGS_IGNORE_ME=yes
DISABLE_IMSETTINGS=yes
# im-config signiture: cce1d9f2a380733e9faadc73e8c273ee -
run_im ibus
, locally on~/.xinputrc
or globally on/etc/X11/xinit/xinputrc
. – Pablo Bianchi Jun 28 '18 at 06:59