Cache Is Not Writable
If you receive the following error when updating the cache in the backend of Magento, you can either delete the file or set the correct permissions on it:
/path/to/magento/app/etc/use_cache.ser is not writable, unable to save cache settings
You will typically only get this error in an environment where PHP is running as an Apache module.
The easiest way to get rid of this error is to simply delete the file. It’s located here:
Another way to fix this is to set the correct permissions on the file:
chmod 666 use_cache.ser
Deleting it is your best bet, since Magento will re-create the file and it will be owned by user nobody.