It seems Blizzard may have pushed the big red button on trading again. Players are reporting being unable to trade and getting the “action unavailable” message when trying it.
The theory is it may be related to boss summon materials duplication, as there have been many reports of it. Duriel summon mats are reported to be the main target, which makes sense since he drops the Uber Uniques.
Blizzard have not issues an official statement on this yet, but they have suspended trading in the past. The trading ban lasted around 5 days last time and you can read more about it here.