Have you had this problem resolved yet?
If not before you replace the Mediator I recommend if it's possible that you also test some other ZIII cards in your 4000T to make sure they are functioning properly, as a faulty motherboard can also cause these symptoms.
My 4000T came out of storage several months ago and it had developed some issues, the IDE was dead, and it wouldn't detect any fast memory. I sent it away to be repaired and the technician recapped the motherboard, fixed a trace, and replaced some blown buffer chips. The 4000T came back with the problems I reported fixed however I then found that ZIII operation didn't work properly while ZII mode for some reason works perfectly. My Meadiator exhibits the same problems as yours, cards are recognised but not functioning, also my Deneb USB card is recognised as ZIII but only works when the jumpers are switched to ZII and using those drivers.
I haven't had time yet to get another round of repairs done on my motherboard, but I was in touch with the technician again and he believes the likely culprit is more faulty buffer chips are a problem with the Buster. I did a Google search at the time and found some others in the past had problems with ZIII on their 4000's attributed to the buffer chips. This of course might have nothing to do with the issues you are having with your Mediator, but's just to make you aware that a messed up motherboard can cause all sorts of weirdness.