Okay, so I have found out why. On the english site the $_POST data sent by the search is completely missing – meaning something clears it before it reaches the search handler. This data is essential to determine which search instance should be used and what the options are.
I have tested to check if the $_POST data exists before plugin loading (in wp-config.php), and it does – meaning it's not the fault of ajax search pro. This means that one of the other plugins completely wipes the content of the $_POST variable. The bad news is, that I literally cannot do anything about this. There is no way to bypass this method or temporary store it. The only way is to find the code responsible of manipulating the $_POST data and either fix it or deactivate it.Best,
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