Sorry for the inconvenient, will investigate regarding to error 402 issue, and will fix asap.
Error 401 and 402 invalid session is about the connection interuption to server.
It may cause by slow connection, too many retrieval access.
You may try to Abort the previous download. Delete temporary download files inside your android firmware download folder, restart Ufi Software, and try again.
But this error is can easy solve by do Retry in ufi downloader menu.
After retry, download will continue without error.