The Yunohosts running in the local network were initially unreachable due to different causes, the cause depending on whether the request was made from the local net or from the public internet.
The local case was easily worked around using the hosts file, by adding a line to /etc/hosts of each device on the local net (and suppressing ensuing headaches):
127.0.0.1 localhost.localdomain localhost 126.96.36.199 online.osba.nl
Once the headaches of trying to edit host files on Android devices promised heavier than those I prospected finding the cause in the local network, I set out finding that cause. It turned out to be DNS rebind protection in the modem/router, an up-to-date (07.01, as of writing) Fritz!Box.
An entry on Marphys blog pointed me to the setting in question:
The list was empty, of course, and after adding online.osba.nl the entries in the hosts files were not needed anymore.