First, is it possible to take another backup? If it is, switch from ZIP to JPA and take a new backup.
You've hit on a bug I found out last week regarding multipart ZIP files (generated whenever there's a non-zero part size). The multipart header overwrites the first file's signature and causes this problem. If taking a new backup is out of the question you can send me the ZIP file and I'll fix the header so you can extract it.
Nicholas K. Dionysopoulos
Lead Developer and Director
🇬🇷Greek: native 🇬🇧English: excellent 🇫🇷French: basic • 🕐 My time zone is Europe / Athens
Please keep in mind my timezone and cultural differences when reading my replies. Thank you!