This happened on one task one slave (all frames worked just fine, and this slave also rendered other tasks without problems):
2013-07-11 17:57:15: 0: Got task!
2013-07-11 17:57:15: 0: Plugin will be reloaded because Reload Plugin Between Frames is enabled for this job
2013-07-11 17:57:15: Constructor: MayaBatch
2013-07-11 17:57:15: 0: Loaded plugin: MayaBatch
2013-07-11 17:57:15: 0: Task timeout is 72000 seconds (Regular Task Timeout)
2013-07-11 17:57:15: 0: Loaded job: [VULCAN] MrX Ingest, converting 13 ABC files for TST_000_0001, \inferno2\projects\vulcan\scenes\TST_000_0001\3d\maya\mrX\2013_06_26 (51df53a0b4e5bb010443b9fd)
2013-07-11 17:57:16: 0: INFO: Executing plugin script C:\Users\Scanlinevfx\AppData\Local\Thinkbox\Deadline6\slave\LAPRO0377\plugins\MayaBatch.py
2013-07-11 17:57:16: 0: INFO: About: Maya Batch Plugin for Deadline
2013-07-11 17:57:16: 0: INFO: The job’s environment will be merged with the current environment before rendering
2013-07-11 17:57:16: 0: INFO: Rendering with Maya version 2012.0
2013-07-11 17:57:16: 0: INFO: Starting monitored managed process MayaBatch
2013-07-11 17:57:16: 0: INFO: Stdout Handling Enabled: True
2013-07-11 17:57:16: 0: INFO: Popup Handling Enabled: True
2013-07-11 17:57:16: 0: INFO: Using Process Tree: True
2013-07-11 17:57:16: 0: INFO: Hiding DOS Window: True
2013-07-11 17:57:16: 0: INFO: Creating New Console: False
2013-07-11 17:57:16: 0: INFO: Enforcing 64 bit build of Maya
2013-07-11 17:57:16: 0: INFO: Render Executable: “C:\Program Files\Autodesk\Maya2012\bin\MayaBatch.exe”
2013-07-11 17:57:16: 0: INFO: Render Argument: -prompt -file “”
2013-07-11 17:57:16: 0: INFO: Startup Directory: “C:\Program Files\Autodesk\Maya2012\bin”
2013-07-11 17:57:16: 0: INFO: Process Priority: BelowNormal
2013-07-11 17:57:16: 0: INFO: Process Affinity: default
2013-07-11 17:57:16: 0: INFO: Process is now running
2013-07-11 17:57:16: 0: Plugin rendering frame(s): 9
2013-07-11 17:57:16: 0: INFO: Executing pre task script C:\Users\Scanlinevfx\AppData\Local\Thinkbox\Deadline6\slave\LAPRO0377\jobsData\prestart.py
2013-07-11 17:57:17: 0: INFO: Waiting until maya is ready to go
2013-07-11 17:57:18: 0: STDOUT: Turned off pyc generation
2013-07-11 17:57:18: 0: STDOUT: Loading config: MayaBatchcfg
2013-07-11 17:57:18: 0: STDOUT: Not found. Trying with all lowercase: mayabatchcfg
2013-07-11 17:57:18: 0: STDOUT: Config path = \S2\exchange\software\managed\pythonScripts\sitecustomize\ScanlineSite\configs\mayabatchcfg.py
2013-07-11 17:57:18: 0: STDOUT: Clearing QT environment variables
2013-07-11 17:57:18: 0: STDOUT: Resetting QT environment variables \s2\exchange\software\managed\Libraries\Qt\4.7.1_x64
2013-07-11 17:57:18: 0: STDOUT: Adding site path //s2/exchange/software/managed/pythonScripts/site-packages
2013-07-11 17:57:18: 0: STDOUT: Adding site path //s2/exchange/software/managed/pythonScripts/site-packages_2.6_64
2013-07-11 17:57:18: 0: STDOUT: Adding site path //s2/exchange/software/managed/pythonScripts/site-packages_configured/sip/windows_amd64_msc9_python26__4_13_2_maya
2013-07-11 17:57:18: 0: STDOUT: Adding site path //s2/exchange/software/managed/pythonScripts/site-packages_configured/PyQt4/windows_amd64_msc9_python26__pyqt4_9_1_qt4_7_1_maya
2013-07-11 17:57:18: 0: STDOUT: Adding site path //s2/exchange/software/managed/pythonScripts/site-packages_configured/scl/windows_amd64_msc9_python26__qt4_7_1_internal
2013-07-11 17:57:18: 0: STDOUT: Scanline Site Initialized
2013-07-11 17:57:18: 0: STDOUT: ======================================================================================
2013-07-11 17:57:19: Exception on Startup: System.Exception: Python Error: Object reference not set to an instance of an object. (System.NullReferenceException)
2013-07-11 17:57:19: Stack Trace:
2013-07-11 17:57:19: at Python.Runtime.MethodBinding.tp_dealloc(IntPtr ob)
2013-07-11 17:57:19: at Python.Runtime.Runtime.PyImport_ExecCodeModule(String name, IntPtr code)
2013-07-11 17:57:19: at Python.Runtime.PythonEngine.ModuleFromString(String name, String code)
2013-07-11 17:57:19: at FranticX.Scripting.PythonNetScriptEngine.ExecuteScript(String scriptName, String script)
2013-07-11 17:57:19: —> System.NullReferenceException: Object reference not set to an instance of an object.
2013-07-11 17:57:19: at Python.Runtime.MethodBinding.tp_dealloc(IntPtr ob)
2013-07-11 17:57:19: at Python.Runtime.Runtime.PyImport_ExecCodeModule(String name, IntPtr code)
2013-07-11 17:57:19: at Python.Runtime.PythonEngine.ModuleFromString(String name, String code)
2013-07-11 17:57:19: at FranticX.Scripting.PythonNetScriptEngine.ExecuteScript(String scriptName, String script)
2013-07-11 17:57:19: — End of inner exception stack trace —
2013-07-11 17:57:19: at FranticX.Scripting.PythonNetScriptEngine.a(Exception A_0)
2013-07-11 17:57:19: at FranticX.Scripting.PythonNetScriptEngine.ExecuteScript(String scriptName, String script)
2013-07-11 17:57:19: at a.a(String[] A_0)
2013-07-11 17:57:19: Deadline Slave will now exit.