You may experience Excel Services crashing continuously after applying a patch to SharePoint 2013.
The issue can be resolved by following these steps:
1. Make sure that the account running Excel Services is part of the groups WSS_WPG and WSS_ADMIN_WPG on the SharePoint servers running Excel Services.
2. Open the SharePoint Management Shell and run the following command:
psconfig -cmd secureresources
3. Run iisreset in the command prompt. This will reset the registry permissions so that the WSS_WPG and WSS_ADMIN_WPG groups have read access to the following registry key:
Excel Services should now run successfully without crashing.