Hi,
I believe that the issue might be related to a similar problem with the most recent plugin version, where the back-end search is accidentally overridden by the plugin, in some rare cases.
There is a temporary fix until the next release:
1. Make sure to have a back-up of everything before editing the files.
2. Open up the wp-content\plugins\ajax-search-pro\includes\classes\filters\class-asp-searchoverride.php file via FTP
3. Replace it’s contents with this.
4. Save the file.
That should solve the issue, hopefully.