I can confirm this happens to me, too. I traded a bunch of times and around every 3rd to 7th time either the accpt button freezes, both players tick accept and nothing goes on, both accept and the trade immediatly gets as done instead of giving one player the 5 seconds delay time or if you place a card into the trade the trade window itself somehow does not fully accept the card and gets stuck. Somehow it removes it from your inventory (clientsided only ofc) and you cant trade with it again. Also the game spits out a system error called: unknown error! (translated from german "Unbekannter Fehler!").
EDIT:
I believe the grouping system could increase the "chance" to let a trade freeze or something.
I remembered that i often traded with people who were in the same group as me. Surprisingly the trade did not go right way more often then usual.