I found the .inis. They are exactly where you described them, just next to the save files. Thought they are stored in the installation folder.
I deleted the .inis and they are recreated both on steam start and when the .exe starts locally.
Will send my new .zip to the person and have him delete the .inis first. Maybe rebuilding the .inis will fix the problem.
Thanks for all the help! Will let you know as soon as I have results.
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The problem has been solved!
The .ini files were the problem. For some reason his .ini files in AppData…\Local were corrupt and didn’t have any InputEvents stored.
So summary:
If errors occur in the shipped version, but not in the development version, this can very well be due to the .ini files. In the development version, the .ini files are located directly in the folder (installation directory), while in the shipped version, the .ini files are located in %AppData%…\Local<UE4 Project Name>\Saved.
So the solution was to delete the .ini files in AppData and have them recreated when the game started.
In addition, I have now packaged and released the game according to Unreal Documentation, i.e. Signing and Encryption. See links from MostHost_LA.
Thank you both so much, you really helped me! 
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Love it when a random hunch pays off 
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