Two lots of ACARS appears in the Inmarsat logs. You'll notice that the first message of each request is different.
The first request is:
17:06:49.406 ... R-Channel RX 0x22 - Access Request (R/T-Channel)
The RX indicates the direction of transmission which means that the aircraft had triggered the sending of ACARS data.
This is followed by a minutes worth of RX and TX messages until the final acknowledge at 17:07:48.907. I interpret this as the period when the ACARS data being sent and accepted. This means that ACARS-over-satellite must have been enabled.
The second request:
18:03:41.405 ... P-Channel TX 0x71 - User Data (ISU) - RLS
In this case the difference is that this is a TX. This means that the request came from the ground. My assumption is that someone (probably ATC) phoned either Boeing or Rolls-Royce to ask if they knew where the aircraft was and this request was triggered by Boeing/RR.
You'll notice that there are no RX messages following the second request so for some reason the aircraft was unable to respond. There are many reasons for a non-reply. It's possible that ACARS was turned off or unpowered. However ACARS-over-satellite needs needs a functioning SDU so it's also possible that this was unpowered.