I believe I might have found the problem in one of the plugin files, I don't think I need the FTP access at all.
To fix the issue, please replace the file contens in wp-content\plugins\ajax-search-pro\includes\classes\actions\class-asp-indextable.php with this: https://gist.githubusercontent.com/anag0/25d119fb40fd1a4ea3d44b6e82822416/raw/5cb9e43bb5bae759ccf5bc10180c638c1f151095/class-asp-indextable.php
After that, make sure to re-create the index table under the Index table options submenu. That should solve the issue permanently. I will also make sure to include this fix in the next release.
PS: please have a back-up of everything, before making any direct changes.Best,
If you like my products, don't forget to rate them on codecanyon :)