Prestashop 1.7.7.7 Unable to update settings in BO/Products after changing Theme

[piskacio]

Fresh install PS v1.7.7.7, and try to add new product. Works fine. Then I changed theme and after adding new product there was error Unable to update settings. Theme developer said that he did not changed Core files just design. I tried installation on another server with different PHP versions and it was the same error. Tried to find solution on forums and nothing works for me.

Screen from Mozilla attached.

There was also this messages

Oops... looks like an unexpected error occurred.

Attempted to call an undefined method named "clearCache" of class "Module".

[Symfony\Component\Debug\Exception\UndefinedMethodException 0]

 

Any idea what to try?

[musicmaster]

As you see the error is a JSON file cannot be parsed. This nearly always means that the JSON file contains a PHP error message instead of the expected JSON structure.

So you need to look in your server's error log what the error was.

Alternatively you can look in your browser's console (network tab) at the contents of the returned JSON file. 

I wrote a post about the later option here: