403 Forbidden error on /wp-admin/admin-ajax.php

I've just migrated a wordpress site to OLS, and I get a 403 Forbidden error on `/wp-admin/admin-ajax.php` by one plugin. I used to have caching enabled but I don't anymore. I used the exact same site with the same plugin on 3 different hosts before, apache and nginx, without a problem.

This only happens with this one plugin.

I've tried following the guide on https://docs.litespeedtech.com/cp/cpanel/403-error/

The permission for the file is 644.

Any ideas about what could be the culprit or how to debug it?
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I also meet the problem. My LSWS mod_security Module is using cwaf_rules_nginx_3-1.233 powered by COMODO. What I settle down way is add /wp-admin/admin-ajax.php into the file named userdata_wl_URLs at the folder cwaf_rules_nginx_3-1.233.