Since there is a time limit to editing post and no way to set it to unlimited (I wonder why, but that’s a question for another day), I’ve set the limit to maximum. There also was an issue, where original “edit” button wouldn’t show up. I did a little research, and turned out that most of the time, it was theme conflict. Changing theme was not an option. So, I’ve just added a custom “edit” link to template. It looks like this
<a href="<?php echo esc_url( home_url( '/' ) ); ?>conversations/reply/<?php bbp_reply_id(); ?>/edit/" class="editbtn"> Edit</a>

It works fine with newer posts. But when I tried editing one of older posts, I got following error:
The mydomain.com page isn’t working
mydomain.com redirected you too many times.

[@robin-w Moderator 8 years, 6 months ago]

not tested but this code should allow editing at all times. Add it to your functions file

add filter ('bbp_past_edit_lock' , 'rew_allow_editing' )

function rew_allow_editing () {
	$retval = false ;
	Return $retval ;
}

on other posts, I’d wonder if the bbp_reply_id(); might be returning a null or the wrong id.

As a test the easiest way would be to replace your Edit </a> with

<?php echo esc_url( home_url( '/' ) ).'conversations/reply/'.bbp_reply_id().'/edit/' ?></a>

That way you’ll see where it is going to send you and can find out what the problem is