This issue may happen on some new updates of browsers, such as Chrome, Safari, etc., due to the enforcement of  SameSite cookie. It will be fixed in the phpFox version 4.8.5

Quick solution

We are going to provide guide to solve this issue on Chrome and Safari browsers.

With Chrome browser

• Open Chrome browser, type on the Address bar: chrome://flags/ 
• Then, search for "SameSite" as the following

• Change the "SameSite by default cookies" flag from "Default" to "Disabled" and Relauch the browser.

• Click Relaunch to reload the Chrome browser.

With Safari browser

• Open Safari, go to Safari > Preferences, then click Privacy.
• Uncheck the Prevent cross-site tracking setting

• Close and reopen Safari.

Now, you can go to AdminCP and can  install or upgrade apps normally.