Ok, then this may well explain the issue. Even based on the INFO messages I'm counting 28 requests (2x13 for the instruments +2 for account and heartbeat). So if there are a few more the 30 per minute is reached. I don't think they put this in the FAQ and don't enforce it. Maybe they relaxed the limit for you individually as you are developing a plugin that has good potential to boost their client base?
I am in contact with IG support, let's see what they respond...