I developed a theme for a customer, and it works locally and on a remote staging server without a problem. Locally is XAMPP Apache, remote staging a linux apache, also on customer side. I need to install it manually by upload as written in the manuals. A first version of the theme worked but after further development and a fresh installation it creates a critical error in WordPress without any hint. This does not happen locally or on my remote test server. The production build was done without a problem. What can be the reason and how can I fix this? It is quite urgent and any help is appreciated.