(This issue may seem identical to several others posted here, but I could not find an exact match, or a solution that would work from the close matches.)
Working in Visual Studio 2015 Enterprise and the latest SSDT/BI tools, running against SQL Server/SSRS 2008 R2, we often find (but not always) that the first attempt to preview or deploy a report fails with this error:
"The definition of this report is not valid or supported by this version of Reporting Services. The report definition may have been created with a later version of Reporting Services, or contain content that is not well-formed or not valid based on Reporting Services schemas. Details: Data at the root level is invalid. Line 300, position 148."
Our workaround for this is to preview or deploy again, and the second attempt always works.
Project Properties -> General -> TargetServerVersion = "SQL Server 2008 R2, 2012 or 2014"
Tools -> Options -> Business Intelligence Designers -> General -> Default Deployment Server Version = 10.5 (this was set to 13, but the experience was the same).
This issue arose almost from the moment we began using Visual Studio 2015; the problem never occured with 2013, but downgrading isn't an option for us.
Thanks to anyone who can help!