Thanks for the quick reply.
I upgraded to 2.9.5.3, but the bug is not fixed for me.
The symptom is as follows.
When I type in affiliate link url and keywords, and click on submit button, it redirects to https://my_site/wp-admin/admin-ajax.php, and I see a blank page which just contains a single digit “0”.
In addition to that bug, I also cannot remove existing links. When I click on removal icon, the clicked link disappears right away via ajax, but when I reload the plugin page, the link is really not gone, and is still there.
So I think ajax interface appears to be broken somehow.
This problem started to occur after I upgraded the plugin to 2.9.5.
Prior to 2.9.5, I was successfully able to add/remove links.