Main repeater transfer fails, ends up in safe mode?

Anyone else run into problems with their main repeaters having transfers failing? I reinstalled Inclusive 12.6 on a Windows 10 machine. Transfers usually work, but sometimes not. Past two times the transfers failed badly enough to push the main repeater (the 1st in a set of 2) into safe mode (the red LEDs flashing).

The only significant change otherwise, from 12.6 installed on a different machine was the replacement of all network switches with ones from Ubiquiti. Instead of past ones that were a mix from various vendors, but the main switch was still one from Ubiquiti. The typical transfer scenario was from a 1gbe wired PC into a Netgear unmanaged switch, with a 1gbe uplink to a 48 port Unifi switch. Linked from there via 1gbe to another unmanaged Netgear switch, with direct connection to both repeaters (into the same switch). Recent changes have migrated out the Netgear switches. First at the PC running Inclusive, and eventually out to the one handling the main repeaters.

There have been no other changes to the network. DNS, IP ranges, upstream ISP connection and firewall configurations have remained the same. Even the wired ethernet cables are still the same ones.

Unifi's console is pretty good at highlighting errors or problems, and none seem to be showing up.

To punt on this situation I moved the main repeaters back onto an unmanaged Netgear switch and used a different PC with a direct connection into that same switch. This time transfers succeeded. There was one change here, the new install has 12.9 on it of Inclusive, not 12.6.

I have not investigated further on-site. Because, y'know, it's no fun telling everyone in the house that the lighting is going offline, especially not when they're in a constant work-from-home scenario.

So I figure it's worth raising the question over here, is anyone else seeing unexpected problems doing what should simple transfers to main repeaters? Especially with Unifi wired managed switches?

My guess is that it has something to do with multicast. In addition to the router, some access points and switches have the ability to block multicast. Just for fun, some manufacturers don't call it multicast.

You can also try using the repeater IP address. Click on advanced in the "find main repeater" screen.