Unfortunately, System UI has stopped I have PAC-ROM Paranoid-Android (downloaded and flashed from here), and suddenly, after changing some settings (unfortunately I can't reproduce an annoying message started popping up every 2 seconds, it says "Unfortunately, System UI has stopped.".

Has anyone faced this before?
Does anyone know how to reproduce the issue or what setting is causing that? I'm sure reflashing the ROM will solve the issue, but I prefer avoiding a reconfiguration of all my personal settings.

UPDATE
I'm noticing now, that since that message started to bug up, the status bar is gone.
This is a symptom of a programming or integration error in the custom ROM. System UI is the name of the process that draws things like the soft home/back/recent buttons and the notification tray. When this process crashes, the soft keys and notification tray, along with anything else it's responsible for, will stop working.

The error could be in their UI customizations, or in something fundamental like the GPU or framebuffer drivers or the kernel. You should report this error to the developers of the custom ROM so that they can fix it.

They'll certainly need more detail to find the problem, such as what device you're using, what settings you changed to make it happen (reproduction steps in technical parlance), and they'll probably ask you for more information too. This answer tells you about getting crash logs using adb logcat and the bugreport command.

Of course, if there's already a newer version of the ROM, the developers will probably not be interested in your bug report unless you can also make it happen on the latest version.