This is even on a new install of dokuwiki!
Turns out php5 changed where utf8_decode was stored (different package). I found it by inspecting the php log at /var/log/apache2/error.log :rolleyes: I think installing php-xml fixed it, but it doesn't hurt to install php-mbstring either.
https://stackoverflow.com/questions/35701730/utf8-endecode-removed-from-php7
sudo apt-get install php-xml php-mbstring