It is nice, that the whole iCloud stuff is just CardDAV and CalDAV. But it is just a pain to set up.
You have to look in /Application Support/AdresseBook/Sources/ID for migration.log.
This files contains the CardDAV Host...
For an example https://p[NUMBER]-contacts.icloud.com/[ID]/carddavhome/card/
Now works and syncs with #thundebird and #kard #aerc (via vdirsync) in #linux.

Technology is so (not) amazing sometimes:

I can send HTML-emails with #Thundebird with inline images from Linux without any problems, but when I attach the same image or forward the mail on with Thunderbird #Windows the images just disappear!

It's not that they're hidden, it looks like they're gone completely (I cannot see them with #SOGO on our mailserver as well).

What #cyberwizardy is this!?