Every time I try to trade someone it glitches and says the trade was cancelled. I have tested this on multiple servers after reading other posts and it still didn't work. I've tested to see if trading two trade evolution Pokemon at the same time caused it and that didn't do anything. We've trade standing still during the trade and that didn't work either. I have tried trading with 5 different people with the same result. Not sure if it is just me or the servers.
This is a known bug and I believe that once a server restarts, the bug is fixed, we are working hard to find a resolution.
Trading doesn't work on most of the servers, it took me 5mins to trade with someone and getting it to work. Hopefully it gets resolved soon but until then we have to be patient.