Using CUPS a random 10% to 30% of all print commands on this printer fail. When this happens the status light on the printer blinks shortly, but nothing is being printed. The print job just vanishes. In all other cases (70% to 90%) the printer just works fine. It doesn't matter what I print, the Ubuntu test page, a short text string, ZPL or a PDF, every time it is 10 to 30% that fails.
I also use an older Zebra GK420d printer, that works flawless using the same driver.

Systems I tested the ZD420 on: Raspbian OS (CUPS 2.2.1), Ubuntu 16.04 (CUPS 2.1.3) and Ubuntu 18.04 (CUPS 2.2.7). I also tried different hardware. On all systems the behaviour was exactly the same. I even tried another ZD420 printer, same results.