If you have received messages about CrashPlan not completing a backup in some time, this can happen because CrashPlan has not been granted Full Disk Access.
Apple introduced this security feature in macOS 10.14 (Mojave), which requires special access to be granted to applications that want access to all the files on your computer. This feature has evolved over time, so even if it used to work, it might need readjusted.
Without Full Disk Access, CrashPlan cannot function because it cannot access all your files, so it can't back them up. If it can't back up everything, then the backup is incomplete. This means the monitoring software sends us alerts, and then we send you email. And followup email. And usually an email after that.
Here's how to grant Full Disk Access:
Go to the Apple menu, and click on System Preferences.
Open Security & Privacy.
Select the Privacy tab.
If privacy settings are locked, click the lock icon in the lower-left corner:
Enter your computer password and click Unlock.
In the left pane, select Full Disk Access.
If you see CrashPlan in the list, check the box next to it:
If you don't see CrashPlan in the list, click the + icon.
Navigate to Applications > CrashPlan.
Click Open.
The following message appears: “CrashPlan” will not have full disk access until it is quit.
Click Quit Now.
Restart your computer.
When it comes back up, log in and look for the 
in the menu bar at the top of the screen. Make sure it appears, that means CrashPlan is running.
Let us know if you have any questions or concerns along the way.