MKDIR error

Using V0.18
cmd: hugo -d \storage\web\extern --verbose --ignoreCache --cleanDestinationDir

2017/01/15 15:03:52 mkdir \storage\web\extern\archive\photo\2016\ein-paar-tage-an-der-ostsee: Das System kann den angegebenen Pfad nicht finden. //System can not find the path

existing path is: \storage\web\extern\archive
dest system is synology NAS 415+
the error happens not allways!

trace:

goroutine 96 [running]:
panic(0xa2cb60, 0xc04281ccd0)
/usr/local/Cellar/go/1.7.4/libexec/src/runtime/panic.go:500 +0x1af
log.Panicln(0xc0426d1a90, 0x1, 0x1)
/usr/local/Cellar/go/1.7.4/libexec/src/log/log.go:334 +0xd0
github.com/spf13/hugo/vendor/github.com/spf13/afero.WriteReader(0xe10820, 0xe68410, 0xc0421fd220, 0x4e, 0xe04d60, 0xc04252fdc0, 0xe06620, 0xc0427d4840)
/Users/bep/go/src/github.com/spf13/hugo/vendor/github.com/spf13/afero/util.go:49 +0x279
github.com/spf13/hugo/helpers.WriteToDisk(0xc0421fd220, 0x4e, 0xe04d60, 0xc04252fdc0, 0xe10820, 0xe68410, 0x0, 0xc042048010)
/Users/bep/go/src/github.com/spf13/hugo/helpers/path.go:585 +0x64
github.com/spf13/hugo/target.(*PagePub).Publish(0xc0424c4880, 0xc0424ed140, 0x3a, 0xe04d60, 0xc04252fdc0, 0x0, 0x0)
/Users/bep/go/src/github.com/spf13/hugo/target/page.go:47 +0x9d
github.com/spf13/hugo/hugolib.(*Site).writeDestPage(0xc0421d4000, 0xc0424ed140, 0x3a, 0x1719a40, 0xc0424c4880, 0xe04d60, 0xc04252fdc0, 0x0, 0x0)
/Users/bep/go/src/github.com/spf13/hugo/hugolib/site.go:1929 +0x166
github.com/spf13/hugo/hugolib.(*Site).renderAndWritePage(0xc0421d4000, 0xc042871f80, 0x14, 0xc0424ed140, 0x3a, 0xaf3ac0, 0xc0421b6000, 0xc0423a5e00, 0xa, 0x10, …)
/Users/bep/go/src/github.com/spf13/hugo/hugolib/site.go:1809 +0x8b9
github.com/spf13/hugo/hugolib.pageRenderer(0xc0421d4000, 0xc042444420, 0xc0424443c0, 0xc042777070)
/Users/bep/go/src/github.com/spf13/hugo/hugolib/site_render.go:73 +0x3e5
created by github.com/spf13/hugo/hugolib.(*Site).renderPages
/Users/bep/go/src/github.com/spf13/hugo/hugolib/site_render.go:46 +0x16a