It’s important to first understand what the “storage metadata” is.
The storage metadata is akin to a MFT (master file table), for the Veeam Backup & Replication backup file. The storage metadata informs Veeam Backup & Replication which blocks within the backup file belong to which file that was backed up. It is stored within the backup file twice for redundancy, and only one entry is ever modified at a time. If both storage metadata entries become corrupt, Veeam has no way to determine which blocks belong to which original files that were backed up.
There are a multitude of potential causes for this issue; the majority are related to either storage failure or a communication issue with the storage device.