PIM is a layer 3 function, so I dont think your 3560 would be interfering with it if its just doing layer 2 between the routers.
The first thing to check would be the routing back to the source. After that, verify that you have an RP for the group, and that the 1811 knows how to reach it. It sounds to me like the 1811 doesnt know where to send the join for the group because it doesnt have an RP defined, or it doesnt know how to reach it.
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...