answer to myself: Cor Nouws wrote on 05-08-16 14:40:
'deb-src http://nl.archive.ubuntu.com/ubuntu/ xenial main restricted'
the 'http://nl.archive' is the problem. Changing to http://archive solves it. I've noted that on the wiki.
https://wiki.documentfoundation.org/Development/Linux_Build_Dependencies#Unable_to_find_source_package_for_.22libreoffice.22_on_Debian.2FUbuntu
-- Cor Nouws GPD key ID: 0xB13480A6 - 591A 30A7 36A0 CE3C 3D28 A038 E49D 7365 B134 80A6 - vrijwilliger http://nl.libreoffice.org - volunteer http://www.libreoffice.org - The Document Foundation Membership Committee Member