after 3 Weeks of uptime with ROS 4.11 the router does not respond to Router Solicitation requests anymore. I defined a input ipv6 firewall rule and the the router does get the packet (icmp type 133) but does not answer.
Debug files go to support@mikrotik.com. That will open up a tech support case for you.
If you want you can paste the information exchanged there regarding the case back here once you have resolution so that other people that in the future might end up searching the forums for the same problem can find a resolution in this thread without having to tie up official support resources.