What happens, in short

"400" error while trying to reply to a Fediverce source, through ActivityPub-enabled blog's account.
Error occurred: rest_invalid_param (400)

Steps to reproduce:

1. from Mastodon Client, log in to a new account. Enter the blog's server. In this case the client was "mona for iPhone" and the server "plusbrothers.net"
2. enter your blog author's username, its own password, then authorize all actions when prompted.
3. search for any fediverse source you're interested in, through the client. Might be any from sports, politics, etc. account.
4. reply through the mastodon client, then send the message.

Scenario where it happened

Let's assume you have an account john-doe@mastodon.social, your main account you interact with, the one with a populated home feed.
You find a post which is interesting for your blog's target, and want to reply to it with your blog's user.
So, you have configured both accounts -the external one and the blog's one- in the same client and you can switch from one to the other.
You open the reply window, you can switch account on the fly. Then you click "send" to continue the thread with your blog's account and it gives the "400" errors.
Here I attach the text file with the log found out from the debug list as instructed in the guide; it also gives a 404 error. I have personalized the 404 page, but not through plugins. The template is modified but no plugin interfers with 404.
enable-mastodon-debug.txt