what price cap means ?
IB implements algorithms to cap the price of your orders before sending them to the exchange. These algorithms have multiple reasons, where the main reason behind them is to prevent fat finger mistakes and potential manipulation. I know that because I used to work in the brokerage industry before and I am working in financial software systems now, so those validations on broker level are very common, however, IB are really over-doing them + in some cases they are implementing them totally incorrect.
I don't know exactly how the algorithms work, but generally speaking they have a -+3% price band around the reference price (Which supposedly gets updated with a certain frequency, so that 3% band is dynamic). If you try to place a buy limit order above that band or a sell limit order below that band, they will cap the order to the band price limit.
With fast moving markets, the price caps does not update as fast and thus it leaves you with either a trade which you can't execute at current market prices or with a position you hold which you can't close at current market price (So you either wait for their band re-adjustment to be able to send your orders or to call them to manually remove the cap for that particular instrument), where both in my opinion are in violation of regulations as they are holding your orders hostage without any valid reason, with or without intention to delay your execution, this becomes a major problem as they are not handling/treating orders in the way they should be.
In my opinion, this is a material breach.