You get this error during feature activation.
If this error occurs, the feature activation code will create an error log file in the temp folder listing the error message. If it can't create a temp folder, the file is written in the drive root.
Most likely the problem is insufficient permissions to update web.config. In order to be able to apply the web.config modification the application pool account running the update must be farm admin. But when you activate feature for some site collection you use an account of the application pool hosting this particular web application. If you follow Microsoft`s best practices for installation and set a different application pool account for each application pool, you will get an error when activating a feature with web.config modifications. This is because the central admin application pool account has specific permissions on stored procedures on the sharepoint_config database,which are required to run the update.
A quick work around for this is to temporarily change the application pool account of the web application from which you will activate the feature, to the same account as the central admin web application. This will make everything work.
Part VII: Troubleshooting Applications
SharePoint Application Deployment Error Messages