SSIS/SSAS 2016 intermittent fault: An error was encountered in the transport layer
I think we solved this or at least found a workaround. 

I believe the problem is a timeout when connecting to SSAS when it has been inactive for some time. It has nothing to do with the actual processing of the cube. 

We added a step to our ETL process which queried the OLAP cube before the processing step was triggered. This causes SSAS to 'wake up' before it is needed.

Since we added this step we haven't had a repeat of this problem.