off topic: can't write or answer PM to some users

Hello!

I don't know how exactly it works, but if the user blocks private messages at the settings page, you won't be able to start a chat with them.

Such as if they set to "No one", you won't be able to message them.

Hello Dirk,

As Prism mentioned,  the Default setting only allows messages from Friends Only, so you need to send a Friend Request to the user, and once they accept, you can PM them.

In your case your setting is set to Everyone, so no need for somebody from somebody in your list to start a conversation with you.

Regards,