The error message is complaining about missing method Validate::isJSON. That's very strange, because this method is clearly there.
There are only two explanations I can think of.
you have modified core class classes/Validate.php and removed/renamed this method. That's very unlikely.
The other possibility is classname conflict. It's possible that some module have class with name Validate, and this class gets loaded at the very beginning of the request. Thirtybees uses lazy loading autoloader, which means it will load any core class only when needed. In this case the core class would never be loaded, because class with the same name exists. But this third party Validate class does not contain isJSON method. This is another way to override core classes. Unlike normal overrides, which extends core class, this method replace the whole core class. Very dangerous, and officially not supported. But some module developers do this kind of things