Probably a routing issue. Shot in the dark would be that one of these routers is NATing traffic, and the other router doesn’t have a route to that NAT’d range.

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tonyarkles a day ago | parent [-]

Other shot in the dark, misconfigured bridging or similar where ARP isn’t getting forwarded and rewritten quite right

	▲	bombcar a day ago \| parent [-]
		Or look for something mangling SRV records, Minecraft seems to use those more regularly than everything else.