This usually happens if the file gets corrupted during upload or extracion. Have you tried to upload the plugin folder manually (with ftp)?
What happens if you deactivate the every plugin and only activate ajax search pro, does the problem persist?
Also, which version of wordpress are you using?
Can you see any error log files (erro_log) in the website main directory or in the plugins/ajax-search-pro directory? If so, then please upload them, I may see some relevant errors in them.
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