Ok, I have now disabled ND with this:
/ipv6 nd
set [ find default=yes ] disabled=yes
With this, IPv6 Router advertisement stops on the affected VLAN.
But why do I need to disable ND? Without disabling ND the Router is doing the IPv6 Router advertisement. But just on one VLAN/Interface! On other VLAN/Interfaces it doesn’t do it with ND enabled. This doesn’t really make sense to me.