I agree with def on the option margin statement. The original problem seems to be that the automated system flatly rejected an appropriate trade. A "covered position" where a credit is received AND the strike is equal to or higher than the margin threshhold (basis of the underlying) should ALWAYS be allowed. The result is that a credit is going to be received and retained by the account holder. If the stock goes up, both the broker and account holder are happy. However, if the stock declines and (for example in this case) 100 shares of the underlying were liquidated to meet the automatic margin call at IB, one Call would have to be repurchased (automatically by IB) to prevent a naked position.
I'm sure this is something IB can structure within their system. Personally, I like to leave a bit of margin room so these situations are less likely to occur.