I have just updated my Lubuntu to the 21.10 (Impish Indri) version and after that the desktop environment is sort of pixelated and laggy in a way which makes it impossible to work with. Some aplications are also displaying this problem, most notably the file manager and also Mendeley Reference Manager, to a lesser extent. This is how my file manager PCManFM-Qt looks like (everything full of pixels and with a delayed response) and this is what happens when I right-click the desktop(the menus are not rendered properly). The problem is also affecting some lubuntu utilities like FeatherPad and KCalc, but Firefox and Thunderbird work completely fine. I sent photos instead of screenshots because when I press print screen, the screen renders correctly and the pixelated anomalies don't show.
Before upgrading everything was normal, so it must be related to something which changed versions in this update. My desktop environment is lxqt and the window manager is openbox. I am not sure if this is relevant, but I am using the Arc-Dark gtk2 theme and I have qt5ct installed. Changing the theme doesn't solve the problem. I am using the correct resolution for my monitor (1920x1080).
A friend of mine, who understands more about Linux than I do, said that probably the problem is with openbox, whose version is 3.6.1-9+deb11u1. I tried to reinstall it using
sudo apt --reinstall install openbox, but it didn't solve the problem. If not with openbox, the problem should be with lxqt, which I also tried to reinstall.
Lubuntu has noted that one needs to copy some configuration file for openbox after the upgrade. I also did this, but nothing changed.
Does someone know what to do?