It looks like mbedtls-2.13.1 tag does not exist from what I can see on github. When I run west update, it is failing on mbedtls even if I have the repo already cloned.
PS C:\Projects\ncs\mbedtls> west update
=== updating fw-nrfconnect-zephyr (zephyr):
--- fw-nrfconnect-zephyr: fetching changes
--- fw-nrfconnect-zephyr: checked out 31529f68ec7fb266a4ec1761009e100ef9850ea5 as detached HEAD
=== updating nffs (modules\fs\nffs):
--- nffs: fetching changes
--- nffs: checked out bc62a2fa9d98ddb5d633c932ea199bc68e10f194 as detached HEAD
=== updating segger (modules\debug\segger):
--- segger: fetching changes
--- segger: checked out 6fcf61606d6012d2c44129edc033f59331e268bc as detached HEAD
=== updating fw-nrfconnect-mcuboot (mcuboot):
--- fw-nrfconnect-mcuboot: fetching changes
--- fw-nrfconnect-mcuboot: checked out 0a5c2987365abb0417610ecb7c2c7b5fb117159b as detached HEAD
=== updating fw-nrfconnect-mcumgr (modules\lib\mcumgr):
--- fw-nrfconnect-mcumgr: fetching changes
--- fw-nrfconnect-mcumgr: checked out c8f675a5fa7f62106b9d913844bd5ed6e16ae69e as detached HEAD
=== updating fw-nrfconnect-tinycbor (modules\lib\tinycbor):
--- fw-nrfconnect-tinycbor: fetching changes
--- fw-nrfconnect-tinycbor: checked out 543ecb7c8662580ef803d59ceda7bd3b8a84a11b as detached HEAD
=== updating nrfxlib (nrfxlib):
--- nrfxlib: fetching changes
--- nrfxlib: checked out a87551ba2e131ff17e0e1c796b73f7c37bbea9ec as detached HEAD
=== updating cmock (test\cmock):
--- cmock: fetching changes
--- cmock: checked out c243b9a7a7b3c471023193992b46cf1bd1910450 as detached HEAD
=== updating unity (test\cmock\vendor\unity):
--- unity: fetching changes
--- unity: checked out 031f3bbe45f8adf504ca3d13e6f093869920b091 as detached HEAD
=== updating mbedtls (mbedtls):
--- mbedtls: fetching changes
From github.com/.../mbedtls
* tag mbedtls-2.13.1 -> FETCH_HEAD
fatal: FETCH_HEAD^commit: not a valid SHA1
FATAL ERROR: Command 'update-ref refs/heads/manifest-rev FETCH_HEAD^{{commit}}' failed with code 128 for mbedtls (mbedtls)