I believe there is some bugs in the Lending API of bitfinex. I was unable to make a new lending offer by POST method to https://api.bitfinex.com/v1/offer/new. Here are the situations.

1. The API documentation stated that insurance_option is optional. However, when this parameter is omitted, HTTP 500 error is returned.

2. I can get a response with insurance_option specified, but the response is always "Invalid offer: not enough balance". I have checked using the wallet balances API to make sure there is enough balance and also make sure that the amount is greater than the minimum lending amount (100USD).