At first, you had to enable "Multicast Support" on ESA0. It´s only possible to enable it on ESA0. Mulicast Assembly at the AP must be enabled (AP Proberties->Advanced->IP Multcast Assembly). You must have the actual firmware, problem with MTU Size is fixed in v8.32.05.007. But in Version 9 the problem still exists, but now it´s fixed in version 9.12.02.0006. You have the fix the channel at the M300 and also at the AP!