You mentioned your rule already works so why not just skip youtube connections in your WEB marking
connection-mark=!YOUTUBE
trust me, i'm no master in this area as well but tinkering with firewall when i bought mikrotik few months ago helped me a lot.thanks for that have tried reading the firewall section of the wiki that just leaves me plenty confused
Thanks for the suggestion. i'm currently observing after i've check my configHi, probably you made yourself a loop in your network (check end-clients, especially switches,and wifi aps),