On certain environments, PHP might have the OPCache extension installed which caches PHP scripts. If you update the plugin, it's possible that a part of the PHP files of the WebARX plugin are still cached and another part is no longer in the cache. This will cause fatal errors.
A solution to this is to restart the webserver and/or PHP on your server.
If this does not fix the issue, please start a new chat with us and provide us with PHP errors from the error logs that are related to WebARX.