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Last tested: Nov 5, 2019
This is likely a timeout error.
If the PDT is persisted with the persist_for parameter then this PDT can only be built by querying it via an explore/look/dashboard and the build is subject to Looker's 60 minute proxy timeout.
If you have determined that this PDT is timing out due to Looker's proxy timeout, consider persisting using a sql_trigger_value instead. PDTs persisted with a sql_trigger_value are generally built by the regenerator and are not affected by Looker's 60 minute proxy timeout.