Here is an update.
I removed my custom fields that were linking to each other.
Now I can see the meta box in the "from" post.
I can see in the admin view of all "to" posts that there is a relation. But when I try to edit the "to" post there is no meta box there.
In settings, the reciprocal field is enabled.
I have set up the meta box in "from" and "to" identically.
What could be the issue ?