I'm also getting the 200 error via API. When placing the order via GUI it gives a friendlier message informing me that my account is ineligible to trade the contract; due to sanctions requirements!??!
And, like you, I'm not having any problems with options on other exchanges that I've tried (e.g. CDE, ASX).
So I'm left wondering... did this just start happening on the 12th? Because it seems possible that, even though on SEHK, are somehow having a ripple effect.
buddy