MIDI Timeout and Mackie MCU Pro. Cannot resolve

I recently added a Mackie MCU Pro to my system. Logic 10.5.1 on a late 2012 iMac, running Mojave, 32GB RAM, no sound card.

The problem? The “infamous” MIDI Timeout error message (attached). As soon as I opened any song the alert would pop up. Also connected to my system is a Korg Nano control 2 and a Nektar Panorama P1, now sold. Both these devices run/ran perfectly, and have never incurred the MIDI Timeout message. I’ve done some searching on the net and have seen this problem, but not matching my issue. There doesn’t appear to be a driver problem here.

So, only now, since adding the MCU Pro, does this annoying, mysterious error come up. Furthermore, it’s causing disruption to my connected El Gato Stream Deck, which also used to perform perfectly. But now, once the dreaded message pops up, the Stream Deck behaves in a random, erratic fashion. Eg: I have a Logic folder containing all kinds of useful shortcuts, etc., but now it refuses to stay in that folder, and “pops out” to the top level, and randomly turns the screensaver on and off. It’s so annoying.

Bu it's not the Stream Deck. Even when this device is disconnected, the error persists.

I’ve tried many things. Safe boot, Recovery Mode, pram reset, discarded preferences, all to no avail. I’ve disconnected everything and only had the MCU plugged in. Still the same. I feel Ive tried everything, but obviously haven't because the message still appears.

Initially Mackie support was very prompt and tried to help, but now they’re not interested, and advised I contact Apple. They didn’t even respond to my last message. They think it's a "setting" in Logic and advised me to contact Logic support. There is no Logic Support. Is there??

So, a 90$ Korg Nano and a 500$ P1 work flawlessly, but an 1800$ MCU cannot and will not.

I hope someone can help resolve this problem, please.

Many thanks for any advice..