This whole situation went on the back-burner for a bit, but we finally got it resolved. I also ended up with tickets on both the Meraki and Cisco side, with the two support techs trying to figure out what was happening. In my case, I'm using Cisco 3850s and Meraki MS425s. The connections would come up fine, negotiate LACP with no errors, then after about 5 minutes, the ports on the Cisco side would err-disable due to a channel problem. We had to issue this command on the Cisco 3850 stack: no spanning-tree etherchannel guard misconfig If I've understood correctly what's happening, once I get all my Cisco 3850 stacks up and running with LACP, then I should be able to turn this back on. STP is behaving in different ways because of the different configurations. Once they are all essentially the same, it should work. . .hopefully.
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