Oops, sorry, you're right. You've enabled the "Upload parts immediately" option. There are 24 parts in total: .jpa, .j01, .j02, ..., .j22, .j23.
First, please use the File and Directory Exclusion feature to exclude the contents of the directory media/com_acymailing/logs from your backup. I don't understand why AcyMailing chooses to put its private log files into a public directory instead of Joomla!'s own log directory as configured in Global Configuration (after all, Joomla!'s log directory is for, well, putting logs in it!) but this doesn't belong in the backup and it's 330 Megabytes. I also suspect that it changes its size while the backup is in progress which explains why the backup gets corrupt and is the exact reason we exclude logs from the backup archives.
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!