Most people receive these error emails starting with 'Warning: file_put_contents(......./Currencies.php): failed to open stream: Permission denied in .....'. This is because the cron stript is executing PHP instead of your web server (Apache, IIS, etc), so different permission will be applied when handling the Currencies.php script. Most people usually just CHMOD their Currencies.php to 777, which basically means anyone can read/write/execute that file. This does cause a security whole but the information that the Currencies.php contains is not sensitive (a user cannot gain access to the site if they were to read this file).
If you are happy CHMOD'ing your file to 777 then please do so as this will solve any permission problems, else try different permission levels, EG: If you know that your account and the web server account belong to the same group then try setting the permission levels to 775.

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