Starting with version 10.4 of Final Cut Pro X, exporting a master file via File -> Share -> Master File... won't work if a LUFS Meter is loaded in Final Cut Pro X. Final Cut Pro X will stop working during export.
This also happens when a LUFS Meter is only used temporarily and no LUFS Meter is present when starting the export. It's even enough to hover the mouse over the LUFS Meter icon in the plugin list of Final Cut Pro X to make exporting fail.
It only occurs under macOS 10.13 with Final Cut Pro X 10.4.0 or later. Earlier versions of Final Cut Pro X are not affected.
An internal routine of Final Cut Pro X stalls while attempting to export. No processing by the LUFS Meter takes place at that time.
Klangfreund and Apple are working together to resolve this issue.
It's worth mentioning that the LUFS Meter passes all the tests in the Audio Unit evaluation tool provided by Apple (auval). It is therefore a fully compliant and well behaving Audio Unit plugin. Every version of the LUFS Meter is rigorously tested (including auval) before it gets released.
Use File -> Send to Compressor instead.