Recently, I have seen the below issue more frequently in which the TFS 2010 to TFS 2012 upgrade failed due to error in analysis services metadata. Below is the error message which I got:
TF255356: The following error occurred when configuring the Team Foundation databases: TF400711: Error occurred while executing servicing step Upgrade Warehouse for component WarehouseDev10ToDev11FinalConfiguration during FinishInstallUpdates: Create Analysis Database schema failed: XML parsing failed at line 1, column 0: A document must contain exactly one root element.
Errors in the metadata manager. An error occurred when instantiating a metadata object from the file, '\\?\C:\Program Files\Microsoft SQL Server\MSAS10_50.IUS\OLAP\Data\Tfs_Analysis.0.db\Team System.2824.cub.xml'
Drop the Tfs_Analysis services cube and restart the upgrade. Still issue persists then navigate to the directory C:\Program Files\Microsoft SQL Server\MSAS10_50.IUS\OLAP\Data\ and delete the content