Could you please check if the settings are saved correctly? This sounds like either a database issue or an unknown incompatibility(?) perhaps.
The line where you get the error merges the default options with the options from the database. So it basically means that the options from the database are incorrect or does not exist.
Could you please check on the database if the tables wp_ajaxsearchpro table exists? If so, then please send me a screenshot of how the data in that table look like.
Could you also send the exact php version you are running? I remember a similar issue from some time ago and it was related to a php module some sort.
Also, if you know how to use the your browsers developer tools, it would be nice to check if the ajax request is not interrupted by anything, and how the header looks like.
I can only hope we can resolve this by guessing. Once you got any temporary online version of it, I'm more than happy to take a look at it.Best,
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