Well, nothing is executed on deactivation, so I can only guess.
- Do you have any custom code in your theme functions.php file or anywhere else that requires the plugin?
- If you activate the error reporting in the wp-config.php file what error message do you get on the front page?
- Were there any modifications made to other plugin files, that may include or require files from the ajax-search-pro plugin?
- If you have any type of cache plugins active, make sure you empty all caches. Some caching plugins use object cache, which might have stored variables and shortcode callbacks from plugins and tries to access the search, yet it's not active anymore.
I'm guessing it's some kind of remaining custom code requesting something from the plugin in the theme header.php or somewhere similar, which triggers a fatal error since the plugin is no longer active.
If you enable the error reporting, I'm sure something will show up. Let me know if nothing helps.Best,
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