I have/had the same issue. This is because the Data Channel defaults to returning a Blob.
I believe the issue is that the blob type cannot be converted to a Uint8Array using the constructor (i.e. Uint8Array(data) as is done in the onDataChannelMessage callback), but rather the async arrayBuffer() method must be used instead, and onDataChannelMessage called by the fulfilment of the promise. (I haven’t tested this way of doing it however.)
A simpler fix is to set the data channel binary type to “arraybuffer”, which will instruct the datachannel to provide an ArrayBuffer rather than a Blob in its MessageEvent.
Add the line datachannel.binaryType = "arraybuffer"; after the datachannel is created in setupDataChannel in webRtcPlayer.js.