Multisite File Permissions Issue

Greetings. I decided to post in here since my web host couldn’t assist me any further on these issues.

I have a network multisite set up on my domain and so far everything were running fine until recently. After upgrading to 4.5, things were fine. However, one of the network sites started to act up after an attempt to upgrade a theme to newer version that I purchased. Before the theme upgrade, everything was fine. Then after upgrading the theme, the problems started, either from theme or from WordPress, which I am not certain yet. I’d appreciate any feedback on these issues and how I can resolve these.

Here are the messages I received within admin panel of one of the network site that is experiencing the problems.

File Permission Issues
We were unable to modify required files. Please ensure that /home/xxx/public_html/wp-content/uploads/ has the proper read-write permissions, or modify your wp-config.php file to contain your FTP login credentials as outlined here..

I did try to add in FTP login information in the wp-config.php but nothing worked. Not sure what else I can do at this point. Let me know if this is server issue related or not.

Then I tried to upgrade my WP to 4.5.1 from 4.5 and here’s the message I got.

I made an attempt to upgrade current WP version 4.5 to 4.5.1 and I got this error message:

Downloading update from https://downloads.wordpress.org/release/wordpress-4.5.1-no-content.zip…

Unpacking the update…
The update cannot be installed because we will be unable to copy some files. This is usually due to inconsistent file permissions.: wp-admin/includes/update-core.php

Installation Failed

Let me know how can I fix this network site. Thanks!