I cannot upgrade/create units because the strategic resources doesn't meet the the requirement even though I met the requirement through trading from other empires. For example, My scouts cannot be upgraded to horsemen because it needs 2 horses. I already have 3 horses as I got 2 from other empire.  Please see attached screenshot.

Is there a chance the trade has been suspended because of violence?

No, I don't think so. I'm able to trade everything. I bought 1 of their horse at first which should have been enough for me to upgrade since I already have 1 horse of my own but I could not upgrade, so I bought another one and still could not do it.