The backup job is configured not to truncate logs, or log truncation is failing due to permission or configuration issues. The disk containing SQL transaction logs becomes full because standard log backups are not happening frequently enough or are not truncating logs after backing them up.
C:\ProgramData\Veeam\Backup\Catalog\Indexes\ veeam backup and replication overflow error
The most common cause is a high amount of data being written to the disk while the backup is active. The backup job is configured not to truncate