The issue has been identified - AWS is experiencing an issue that in turn is making some Nucleus services error or respond poorly.
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From their status page: Starting at 7:46 PM PDT we experienced higher error rates and latencies for AWS services within the US-EAST-1 region. These error rates and latencies have seen some improvement from 7:55 PM PDT, but remain elevated. We continue to work towards mitigating the impact and will continue to provide updates as we progress.
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We are taking action to mitigate possible fallout from errors by pausing processing of transactions etc. during this outage. They will resume normally once the underlying issue is fully resolved.