I think I have found a workaround to this problem.
When YH is started or rebooted:
- stop yunohost-firewall
- start coturn-synapse (if not started) or restart
- start yunohost-firewall
It looks like this also works:
- start coturn-synapse (if not started) or restart
- stop yunohost-firewall
- start yunohost-firewall
Someone who has any idea why this works?
edit: restart yunohost-firewall seems not to work.