I proudly created an small 'update_post_meta' function which works exactly the way I need.
...I thouhgt, now I realize that using the inline editing feature of AdminColumnsPro, the update function does not work.
It's the feature (inline editing) of the plugin Admin Columns Pro and I recommend contacting ACP support to ask for help with this issue. They will know how the feature works and give a better solution.
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