Hi,

Based on ( https://msdn.microsoft.com/en-us/library/ms160340.aspx ), when use RSCM to change service account, it will automatically updates the login permissions on the SQL Server Database Engine instance used to host the report server database. The new account will be added to the RSExecRole. So please change service account to a available one in RSCM.

Since you mention you get the error message, please check the SSRS log files(C:\Program Files\Microsoft SQL Server\MSRS10_50.MSSQLSERVER\Reporting Services\LogFiles) to find detail error information.