After removing the > 5GB Windows image per request, and setting the timeout to 60, a different trace is appearing (still with that lock file not existing):

http://paste.ubuntu.com/13833338/

2015-12-08 20:10:34+0000 [-] Unhandled error in Deferred:
2015-12-08 20:10:34+0000 [-] Unhandled Error
        Traceback (most recent call last):
          File "/usr/lib/python2.7/threading.py", line 783, in __bootstrap
            self.__bootstrap_inner()
          File "/usr/lib/python2.7/threading.py", line 810, in __bootstrap_inner
            self.run()
          File "/usr/lib/python2.7/threading.py", line 763, in run
            self.__target(*self.__args, **self.__kwargs)
        --- <exception caught here> ---
          File "/usr/lib/python2.7/dist-packages/twisted/python/threadpool.py", line 191, in _worker
            result = context.call(ctx, function, *args, **kwargs)
          File "/usr/lib/python2.7/dist-packages/twisted/python/context.py", line 118, in callWithContext
            return self.currentContext().callWithContext(ctx, func, *args, **kw)
          File "/usr/lib/python2.7/dist-packages/twisted/python/context.py", line 81, in callWithContext
            return func(*args,**kw)
          File "/usr/lib/python2.7/dist-packages/provisioningserver/utils/twisted.py", line 200, in wrapper
            return func(*args, **kwargs)
          File "/usr/lib/python2.7/dist-packages/provisioningserver/rpc/boot_images.py", line 118, in _run_import
            reload_boot_images()
          File "/usr/lib/python2.7/dist-packages/provisioningserver/rpc/boot_images.py", line 57, in reload_boot_images
            CACHED_BOOT_IMAGES = tftppath.list_boot_images(config.tftp_root)
          File "/usr/lib/python2.7/dist-packages/provisioningserver/boot/tftppath.py", line 266, in list_boot_images
            extract_image_params(path, metadata) for path in paths))
          File "/usr/lib/python2.7/dist-packages/provisioningserver/boot/tftppath.py", line 266, in <genexpr>
            extract_image_params(path, metadata) for path in paths))
          File "/usr/lib/python2.7/dist-packages/provisioningserver/boot/tftppath.py", line 180, in extract_image_params
            arch, subarch, release, label)
          File "/usr/lib/python2.7/dist-packages/provisioningserver/drivers/osystem/custom.py", line 58, in get_xinstall_parameters
            with ClusterConfiguration.open() as config:
          File "/usr/lib/python2.7/contextlib.py", line 17, in __enter__
            return self.gen.next()
          File "/usr/lib/python2.7/dist-packages/provisioningserver/config.py", line 674, in open
            with cls.backend.open(filepath) as store:
          File "/usr/lib/python2.7/contextlib.py", line 17, in __enter__
            return self.gen.next()
          File "/usr/lib/python2.7/dist-packages/provisioningserver/config.py", line 549, in open
            with RunLock(path).wait(timeout=60.0):
          File "/usr/lib/python2.7/contextlib.py", line 17, in __enter__
            return self.gen.next()
          File "/usr/lib/python2.7/dist-packages/provisioningserver/utils/fs.py", line 441, in wait
            raise self.NotAvailable(self.fslock.name)
        provisioningserver.utils.fs.NotAvailable: /run/lock/maas.L2V0Yy9tYWFzL2NsdXN0ZXJkLmNvbmY=.lock