For link "Like" buttons

.....topic/159359/?do=reactComment&comment=3821287&reaction=1&csrfKey=86e3767b7fd93453c821ec55c26484e7

When i press Like server returns "404 not found"

That link appears to be malformed, there's only the Topic ID without any title after it.

topic/159359/?do=reactComment.......(etc)

but the link should contain the topic title.

e.g for this topic, and your post:

topic/449955-how-to-find-ajax-handler/?do=reactComment......(etc)

Have you customized your FURL's, or are running a 3rd-party plugin/app which changed the FURl structure?

 

I use furl without -{?} part. 

Handler for ?do=reactComment... (Reaction) placed in system/Content/Reactable.php

In my case there was an error in DB table structure (core_reputation_index) after upgrade from 3.2 to 4.3.6 and engine return error 404 (!).  It absolutely is intended (c) Bfarber

By the way: ACP check tool did not see errors in table structure.

What was the error ?

ALTER TABLE `ipb`.`core_reputation_index` CHANGE `member_received` `member_received` BIGINT(20) UNSIGNED DEFAULT 0 NOT NULL; 

ALTER TABLE `ipb`.`core_reputation_index` DROP COLUMN `misc`, DROP COLUMN `rep_for`, CHANGE `app` `app` VARCHAR(32) CHARSET utf8 COLLATE utf8_unicode_ci DEFAULT '' NOT NULL, CHANGE `type` `type` VARCHAR(32) CHARSET utf8 COLLATE utf8_unicode_ci DEFAULT '' NOT NULL;