Exception:

An error occurred within the report server database. This may be due to a connection failure, timeout or low disk condition within the database. For more information about this error navigate to the report server on the local server machine, or enable remote errors

Workaround:

Changing the data files 'auto growth' property from restricted to unrestricted resolved the issue.

• Connected to the database server thru Microsoft SQL Server Management Studio
• Right click on the database
• Select properties
• Select files
• Click on ellipses buttons
• Select "unrestricted File Growth"
• Click OK

Exception:

Login failed for user XXX

Explanation:

I am not able to connect to TFS cube even though, I am in Tfs_Analysis --> TfsWarehouseDataReader's role. 

Solution:

Make sure that you have enabled the "Read definition" database permission for TfsWarehouseDataReader role

Exception:

After upgrading to TFS 2013, the TFS OLAP Cube fails processing with one of the following messages:

Invalid column name ‘System_AssignedTo__Name’.; 42S22;

Invalid column name ‘System_AssignedTo__SID’.; 42S22

Query execution failed for dataset ‘ds[Whatever]‘

Solution:

• In TFS Admin Console, look up the account under “Account for accessing data sources” under the “Analysis Services” tab (Reporting » Edit).
• In SQL Server, add that account to the Tfs_Warehouse database, to the TFSEXECROLE role.