I don't know the “Zen” browser and don't use it; consequently, I haven't tested NZB Donkey on this browser. Therefore, I cannot provide support for using NZB Donkey in the “Zen” browser.
Only the latest versions of Firefox (or Chrome) are tested, and only these are officially supported.

If you can reproduce the same error in Firefox (or Chrome), please open an issue on https://github.com/Tensai75/NZBDonkey

Thanks and best regards, Tensai