Hi,
Thank you for the details, I logged in and found the issue.
Actually it was not only a settings related problem. The PHP json_encode function simply failed when tried to convert the results to the script. I looked it up, and there was a single non UTF-8 character cutoff within one of the queried fields, and that basically completely made the JSON encoding fail. I have added a line of code to fix that, it was luckily nothing too complicated. I will make sure to include this fix for the upcoming release. This never came up in the past 10 years, so there is always something new to fix.
As for the settings, I have disabled the search cache as well as the custom ajax handler. You can enable the cache later, but part of the inconsistent results was, that the plugin was actually able to display some of the cached result sets for different queries, when others already failed due to the JSON encoding.