The final error message is important there.
It indicates that the system runs out of memory when trying to load the active theme. Most likely it was already on the edge of running out of memory, and by adding 1 more plugin it broke.
Ajax search pro does not load or execute anything on the admin back-end, unless you are exactly on its settings pages, to prevent any compatibility issues and to increase performance, so it's most likely not related to this exact plugin.
You can try increasing the memory limit programatically by editing the wp-config.php file in your wordpress folder, by adding these lines:
ini_set('memory_limit','512M'); define('WP_MEMORY_LIMIT', '512M');
512 MB should be more than enough, but of course it depends on the installation (plugins count, theme etc..)Best,
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