Starting with version 10.4 of Final Cut Pro X, exporting a master file won't work if a LUFS Meter is loaded in Final Cut Pro X. Final Cut Pro X will hang.
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 and not on earlier versions of macOS.
An internal routine of Final Cut Pro X stalls while attempting to export. No processing by the LUFS Meter takes place at that time.
It's worth mentioning that every version of the LUFS Meter gets tested with the Audio Unit evaluation tool provided by Apple (auval). A new version of the LUFS Meter only gets released to the public after it passes this set of tests (and others).
After reporting this to Apple, the developers of Final Cut Pro X were really quick in finding a fix for this issue.
This fix isn't yet released as part of an update of Final Cut Pro X though.
Until the fix for Final Cut Pro X is released, this workaround is recommened to circumvent the issue: