Looks like this is what failed
[18/01/2016 - 11:59:35 PM] Backing Up File: C:\Users\DefaultAppPool\AppData\Local\Microsoft\Windows\UsrClass.dat
[18/01/2016 - 11:59:35 PM] Result: Failed - Error: -1 (API Reg Save Failed (), Tried File Copy, File In use, Cannot copy.)
[18/01/2016 - 11:59:35 PM] Backing Up File: C:\Users\DefaultAppPool\ntuser.dat
[18/01/2016 - 11:59:35 PM] Result: Failed - Error: -1 (API Reg Save Failed (), Tried File Copy, File In use, Cannot copy.)
If using the fallback method that failed because the registry files where in use, looks like those are for a program so you can just uncheck those in the backup to have it skip them or if ou want them backed up you will need to use VSS instead of the fallback method. But those registry files are not very important so you should be fine with just skipping them
C:\Users\DefaultAppPool is related to IIS if you have it installed.
Shane