The XML for Analysis request timed out before it was completed.

I have experienced the same.

Whats the time between the start and the end of the refresh?

My model took more than 2 hours to refresh (if succeeding at all), and I found out that the error was due to a query in the model, that took up a lot of ressources - ultimately resulting in a time out.

I went through the datamodel to check all queries and measures, and found a calculated column referencing data in 2 tables that took forever to process. 

Maybe this could be the issue in your case.