I realize this is the channel for the datetime plugin, but there doesn't seem to be a channel for the clock plugin, and they are related. If you create a new clock plugin and then go to Properties>Clock Options>Format there is a dropdown with 4 presets and the option for a custom format. If you select one of the presets and close the window it will save as a blank custom format.
There are multiple people being affected by this: https://askubuntu.com/questions/959339/xfce-panel-clock-disappears
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