Windows server backup fails with these symptoms: 

In the Windows Server Backup GUI, when you double-click a failed backup:
"Failed. The system writer is not found in the backup." 

After running "vssadmin list writers", System Writer is not listed and these
two errors appear in the Application event log:

--------------------------------------------------------------------------------
Event ID 8193
Volume Shadow Copy Service error: Unexpected error calling routine XML
document is too long.  hr = 0x80070018, The program issued a command but the
command length is incorrect.

Operation:
   Writer Exposing its Metadata

Context:
   Execution Context: Requestor
   Writer Instance ID: {6B443A2A-3ABF-4DF7-87B3-C13B120D0DD2}
   Writer Class Id: {E8132975-6F93-4464-A53E-1050253AE220}
   Writer Name: System Writer
--------------------------------------------------------------------------------
Event ID 8193
Volume Shadow Copy Service error: Unexpected error calling routine
CreateVssExamineWriterMetadata.  hr = 0x80042302, A Volume Shadow Copy Service
component encountered an unexpected error.
Check the Application event log for more information.

Operation:
   Writer Exposing its Metadata

Context:
   Execution Context: Requestor
   Writer Instance ID: {6B443A2A-3ABF-4DF7-87B3-C13B120D0DD2}
   Writer Class Id: {E8132975-6F93-4464-A53E-1050253AE220}
   Writer Name: System Writer
--------------------------------------------------------------------------------


Fix:

Various parts of Windows Server still limit path depth and number of
subdirectories per directory. The temporary dirs in this location exceed those
limits for the VSS writer:

C:\Windows\Microsoft.Net

Find out which subdirs in this structure are big. Then, stop IIS, move the
contents of those temp dirs (specifically, move the 'root' subdir out of
"Temporary ASP.NET Files"), then start IIS again and verify that web apps
still run. If web apps still run, it is OK to delete the moved 'root' folder.

see also:
https://support.microsoft.com/en-us/kb/2807849
http://blogs.msdn.com/b/ntdebugging/archive/2013/08/27/missing-system-writer-case-explained.aspx

note: this failure mode has similar symptoms, but is not the same cause as the
above:
https://support.microsoft.com/en-us/kb/2009272