This article describes the cause and possible solutions for the screenshot verification errors "(file) is missing or corrupt" or "BOOTMGR is missing."
Figure 1 - Example of the error "BOOTMGR is missing"
Figure 2 - Example of "BootMGR is missing or corrupt" error message
Although these types of errors can have many different catalysts, the backup agent may just need to reconcile its current backup with a new full scan of the target machine.
Manually force a differential merge for the agent (Figure 3). Note that this will force the agent to traverse the entirety of each volume being backed up, giving the appearance of a new full backup. However, a differential merge backup will only incur a cost of space on the storage array equal to a normal incremental and a minor amount of overhead.
Figure 3 - Force a Differential Merge from the Configure Agent Settings of the web UI