MSCHAPv2 fails to authenticate against OpenDirectory with error 5100 (0x13ec)

[Alan Egerton]

Dear FreeRADIUS users,

I'm trying to follow the instructions in Dan Barrett's article
<https://www.yesdevnull.net/2013/10/os-x-mavericks-server-setting-up-freeradius/>
to get the distribution of FreeRADIUS (v2.2.0) that ships with OS X
Server (v4.1) to authenticate Open Directory users via non-Apple
authenticators.

However, the MSCHAPv2 module always fails as follows:

[mschap] No Cleartext-Password configured.  Cannot create LM-Password.
[mschap] No Cleartext-Password configured.  Cannot create NT-Password.
[mschap] Creating challenge hash with username: username
[mschap] Client is using MS-CHAPv2 for username, we need NT-Password
[mschap] Using OpenDirectory to authenticate
[mschap] Doing OD MSCHAPv2 auth
[mschap] Authentication failed for username: error 5100 (0x13ec): unknown error

I can't find any documentation on what might have caused this error
and am not sure how to progress from here.  Your thoughts would be
most welcome!

Kind regards,
-- Alan