I'm guessing you have already checked it, but is the View Composer service running? I had a strange issues happen a couple of times on 5.1.2 where the composer service would die and when restarted would fail again as soon as provisioning was enabled on a specific pool. Turned out that there was a snapshot that composer was having issues with. The composer log would show an error related to the snapshot, but I can't recall specifically what the error was. The fix was to create an additional snapshot on the template and then recompose the pool from the new snapshot.
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