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Last tested: Nov 5, 2018
This error means that you're running with the new encryption and is using a locally backed key. The provided key doesn't match the format that we expect, in this particular case we detected the key was base64 decoded it and the base64 decoded output wasn't the proper number of bytes, which should be 32.
A recommended way to generate keys is with the following:
>openssl rand 32 -base64 > key_file