Could you please provide tamporary FTP access as well? Without debugging the final query and the options I'm not able to tell where the problem might be.
The configuration looks all right to me so it's something else. There was no content when I enabled filling options as well, so my next guess is that the table prefix might be incorrect within the query, but I can't confirm nor fix unless I can check the code, sorry about that.
I remember someone else had a similar issue with ajax search pro + wpengine. It was a one time issue, but it might be it.
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