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Of course, if there's already a newer version of the ROM, the developers will probably not be interested in your bug report unless you can also make it happen on the latest version.