Thank you, this was really useful information! It turns out that Ninja Forms was using the WP_Session class just like Ajax Search Pro in the latest version, but they have probably a very old version.
I had to make a change in the ajax search pro plugin code to apply a quick hotfix for this issue. Now they should work correctly. Thank you for the proper information.Best,
If you like my products, don't forget to rate them on codecanyon :)