Hi Daniel, thanks for responding.
This problem seems to be very intermittent. It happened several times a few months ago, and my music colleague who also owns another UFX+ has had it happen a few times too.

On this occasion the loss of Thunderbolt connection seemed to happen when I removed a USB cable that was feeding midi to an interface (Emagic amt8 interface). For several hours there was nothing I could do to get Thunderbolt to connect with the MacBook Pro again. I tried rebooting many times, rebooting the computer, swapping to another computer etc. I switched over to using USB temporarily that day for the audio connection, but this is not ideal in my setup. Later in the evening the UFX+ just randomly connected by itself over Thunderbolt again.

I had wondered if the MacBook itself was losing the channel to the Thunderbolt socket somehow, but the problem persisted on both Thunderbolt sockets and on another MacBook too, so I have to assume that the UFX+ itself is somehow disabling its own Thunderbolt connection for some reason.

I should add, when this happens, the option to select "Thunderbolt" on the UFX+ screen is actually "greyed out" as if the option for Thunderbolt isn't installed at all.

At the moment the Thunderbolt connection seems to be behaving properly again every day, but as I've seen this happen a few times in the past I expect it to happen in again at some point, so I thought it would be best to notify you here, and also for the benefit of other users who might search this forum for the same problem.

Kind regards