AnalysisServices: The connection is not open.. Microsoft.Data.Mashup.ErrorCode = 10478

Hi Max_WH 

My support case for this error message is almost ready to be closed.  I thought I'd let you know.  It turns out there was a bug in the way the PQ mashup engine interacts with the underlying connections to the PBI dataset.  This behavior should be documented by Microsoft in the near future, and I'll post a link here.

I opened two SR # for this bug:
2503220040000275
and 

2504010010001220

The second was a unified/premier case.

I'm not totally sure, but I think your case and my case were both associated with the same internal IcM (Incident-603360591)  You should be able to confirm that with your Mindtree engineer, if you provide my IcM number.

I have an RCA and a longer explanation can be found here:
https://community.fabric.microsoft.com/t5/Service/DataSource-Error-AnalysisServices-The-connection-either-timed/m-p/4702221/highlight/true#M276330

The RCA for my case is like so:

I don't know if that helps.  At the end of the day, the workaround is pretty straightforward.  You have to wrap any queries for the AnalysisServices connector, and put Table.Buffer around them.  This ensures that connections won't be held open for long periods of time resulting in socket exceptions.  In my case the offending connection was being held open for internal reasons that are NOT in my control.  It has something to do with the fact that we are using one query as the driver to loop/execute some nested queries.

Hopefully this is makes sense. Your best is to test with Table.Buffer and see if that avoids the error.  IMO this is an obvious bug, but the ASWL PG doesn't currently see it that way (possibly because the bug isn't theirs but belongs to a related product team - PBI integrations or whatever).