@cbring, did you read the issue you linked? The problem is in the library we use (as described in the issue) and a bug was filed against this library (RTU over TCP error: response data size '5' does not match count '4' · Issue #52 · grid-x/modbus · GitHub).
There is nothing we can do from the Telegraf side to fix this, all we can do is to wait for the project to fix the issue. So if you want to help, you can open a pull-request on grid-x/modbus with a fix (see my comment for ideas)…
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