I think the problem is that your key export fails, because you pointed
--homedir at the (presumably) empty directory "%tmp%\_tempKeyring".

The export did not use any filter and tried to export a key as can be seen in
Msg8313 "error receiving key from agent"
The import itself also stated no errors as it can be seen in T2355 (dranft on May 12 2016, 03:00 PM / Roundup), but this
imported secret key cannot be used (or exported) anymore.
Also important: This is no longer reproducible in 2.1.14 (which might be enough
to set the bug to fixed)

PS: forget the --homedir thing, it is even reprodicable in the default folder in
%appdata%.

Sorry, forgot my import cmdline:

C:\Program Files (x86)\GNU\GnuPG\2.1.12\bin>gpg --batch --homedir
%tmp%\_tempKeyring --import "P:\2EEC2B65A2B4B3EF.sec.asc"
gpg: Die "Keybox" `C:/Users/ranftd/AppData/Local/Temp/_tempKeyring/pubring.kbx'
wurde erstellt
gpg: C:/Users/ranftd/AppData/Local/Temp/_tempKeyring/trustdb.gpg: trust-db erzeugt
gpg: Schlüssel A2B4B3EF: Öffentlicher Schlüssel "Daniel Ranft (Giegerich &
Partner GmbH)" importiert
gpg: Schlüssel A2B4B3EF: "Daniel Ranft (Giegerich & Partner GmbH)" nicht geändert
gpg: Schlüssel A2B4B3EF: geheimer Schlüssel importiert
gpg: Anzahl insgesamt bearbeiteter Schlüssel: 4
gpg: importiert: 1
gpg: unverändert: 1
gpg: gelesene geheime Schlüssel: 3
gpg: unveränderte geh. Schl.: 2
gpg: keine ultimativ vertrauenswürdigen Schlüssel gefunden

damn, thats the creation date of the last userid - i was too stupid to read the
draft of hkp.
http://tools.ietf.org/html/draft-shaw-openpgp-hkp-00#section-5.2
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