This is the response from the Bitfinex team: The only requirement for the nonce is that it has a larger value than the previous nonce. The code used to generate a nonce used looks like it would produce a larger number each time, but perhaps a greater nonce was inadvertantly entered at some point. Is there anything else in the code that may conflict with this method of generating a nonce? Are there multiple processes running using the same API key? Due to the fact that the nonce needs to be increasing each time, if you are running multiple concurrent integrations it is recommended to use a different API key for each one.
The nonce that is generated in the library is by design bigger after each call. There should be no reason why it would suddenly not be bigger than the last. Do you maybe run some other application or 3rd party app which uses the same API key? The nonce is bound to the API key, so if you're using it for multiple purposes they might interfere with eachother.
Нежелательно использовать один API key в разных программах. Будет периодически проблема Nonce is too small при выставлении заявок.