I've been selling AP resets (like 7 full trades in a row), then I hit the meso limit. So I bought 2 billion meso coins and then tried to sell more AP resets, but we kept getting the error message. I logged out then back in and the error persisted. I'm not sure if there is some hidden restriction on trading or what, but I thought I'd bring this up.
It can happen anytime, I don't think it's related with reaching the max meso limit. There is also no restriction on trading them, but I believe this problem is caused by a glitched cash item (like, one of those APRs you buy has a nasty bug inside?) This problems occurs to many, often. Andreas has explained how to fix it (in this thread: https://royals.ms/forum/threads/unsuccessful-trade.115863/), so I'm quoting him here