Connect S4048F (OS9) and S5248F (OS10) via 40Gb QSFP

I've got a pair of "old" Top of Rack switches (4048's) that I'm trying to link with a pair of new ones (5248's) with Dell branded 40Gb QSFP Cables to allow for a gradual migration of connected hardware. When I connect even a single cable the port, the interface comes up (now that I've added the line "intf-type cr4 autoneg" to the 4048 interfaces - that took some finding!) but immediately goes into a spanning tree "blocking" state and no traffic is passed - regardless of config.

The end game to have two cables in a Trunked Port Channel to handle the required VLANs, but we've simplified it down to a single cable that's configured as an access port on each end for a VLAN that doesn't exist anywhere else on the network, and it still blocks all traffic?

Any ideas?