I have restored the plugin, and activated it. Funny enough, the old modifications were still there in the functions.php file in the theme directory, including the last one. I've noticed it once the plugin was already active, but it didn't cause any error pages or missing page messages.
Nevertheless, I've made a change to that code, a different approach, but I have still no idea what exactly the cause was, as it did not re-appear once the plugin was activated, and I believe it should have. So far it looks like it's working, I don't see any error messages.
Once thing I have noticed is, that there is some sort of database optimization plugin, which does some changes on the database table contents, including the ajax search pro database tables. If possible, I would recommend turning it off for the ajax search pro tables. The search instance information stored there is serialized and very fragile. Any sort of unexpected change could cause data corruption, and while I designed the plugin to handle those situations, but I still do not recommend that. I don't think it actually does any harm at all, but since the issue is still unexplained, I want to rule out any possible 3rd party plugin involvement.Best,
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