Same issue here after upgrading from 6.3.x to 7.0.x (now running 7.1-6 and Backup Server 2.1-2), some backups are failing due to "timeout" issues
All Proxmox Server have FC NVMe Storage, Backup Server will write to NFS mount (QNAP)
Note: there was no issue at all while being on Proxmox 6.x...