Theme does not work on customer server but on local and on remote test server

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.

Many thanks,

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