The reason we're choking on these frames is because they're being sourced from a multicast MAC address. The LSB of the first byte of the MAC address is the multicast flag. If you're looking at a hex MAC address, you know it is multicast if the 2nd number from the left is odd. In your case, the 2nd number in is 5, which indicates it is multicast.
Since the switch wont install multicast/broadcast MAC addresses in the CAM table, we throw this error. Since it tell us which port the invalid packets are coming from, it gives us a direction to troubleshoot.
If the Dell switch is a managed switch, i'd look at 2 things:
1. The event log to see if they run the same checks we do on ingress frames
2. If nothing is in the event logs, maybe it is programmed to learn these mac addresses. I'd look at the mac-address table of the Dell then to see which port we're learning it on.
If the switch is unmanaged, or the 2 steps above dont give away the position of the funky source, I would set up a SPAN session of 4/2 on the 4k and send it to a port connected to a sniffer. Once I was sniffing the traffic, I would start yanking cables on the Dell switch until these packets went away.
