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Relevant error:

2023-01-30T17:30:26.097Z - error: Error occurred while backfilling events for opts.uri=undefined gitterRoomId=556b681e15522ed4b3e1357f: RethrownError: Failed to import gitterMessageId=5775b3f83da1a2ce671cef1d
    at ./modules/matrix-bridge/lib/gitter-to-matrix-historical-import.js:396:18
    --- Original Error ---
    StatusError: sendEventAtTimestmap({ matrixRoomId: !JXttTWkIIhVgFnffTO:beta.gitter.im }) failed 400: {"errcode":"M_UNKNOWN","error":"'body' not in content"}
        at _sendEventWrapper (./modules/matrix-bridge/lib/matrix-utils.js:945:31)
        at runMicrotasks (<anonymous>)
        at processTicksAndRejections (internal/process/task_queues.js:93:5)
        at async MatrixUtils.sendEventAtTimestmap (./modules/matrix-bridge/lib/matrix-utils.js:972:17)
        at async ./modules/matrix-bridge/lib/gitter-to-matrix-historical-import.js:357:27

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Follow-up to !2313 (merged)

Part of #2609 (closed)