Hi Ilya,
For Standard HLLAPI use 12 bytes for a session; and for Enhanced HLLAPI use 16 bytes. When querying max 24 sessions, on input the Length parameter should be 24 times the session size (288 or 384), and the byte array parameter needs to be at least this size. On return the byte array will contain session info of any actively running Passport sessions queried, and the Length parameter will be the count of sessions found. Does this help? If your input parameters were correct, do you have Passport sessions running?
Thanks,
Mike