Usually this is a client side issue. Invalid MDIE, happens when a client claims to support Fast Transition, but sends a wrong MDIE. The PMKID is generated as part of the Authentication process, based on the exchange between the AP and the Client. If a client is connected to the wireless network and onboarded, a PMKID is already generated, and is also used during roaming. If Invalid PMKID happens, it either means that the client is sending a wrong PMKID, or it is unknown to the AP. In any case, Fast Transition might've gone wrong, and usually this is a client issue. I'd look into if there's a software upgrade available for the client. Historically, wireless clients have been having lots of issues with Fast Transition, but I'd argue that was a thing in the past. Modern wireless clients have much better support for standards-based roaming. That being said, avoid setting Fast Transition to Supported/Optional/Adaptive. Either enable it fully or disable it. I've seen more issues when FT is set to Adaptive, rather than it being Enabled or Disabled.
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