I have tried to update to the latest version of Mantra and got this error message;
An error occurred while updating Mantra: The update cannot be installed because we will be unable to copy some files. This is usually due to inconsistent file permissions. js/PIE/error_log
The error mostly says it all: you have problems with file permissions. Correct those to be able to install/update extensions.
The website files should be owned by the webserver user (or whichever user the webserver runs under for this particular account) to avoid this kind of error.