Please switch to Parent theme, deactivate all plugins (except Required Plugins) and then reactivate them one by one. This makes it very easy to isolate the plugin causing problems and double check.
But I can’t update the theme on this test domain (I need to have the same version to find where is the issue) because the licence is already taken by the online website.
There is no way to have the licence work for a dev version of a website?
1 license key can be used for the live and dev (staging) site at the same time. When activating the license for the dev site, just check an appropriate checkbox and activate the same license as you have for the live site: https://prnt.sc/Yo976mxQ5nWE