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

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

I don't believe this demonstrates a bug.

I think the problem is that your key export fails, because you pointed --homedir at the (presumably)
empty directory "%tmp%\_tempKeyring". This leads to the not very helpful error message about the
eof. If the export were successful, gpg would have written the key to stdout.

For reference, here is what I tried. First GNUPGHOME points to a home with the key I want to export:

    $ echo $GNUPGHOME
    /tmp/tmp.T7I4M9RIc3
    $ g10/gpg --list-keys alpha
    gpg: please do a --check-trustdb
    pub   dsa1024 1999-03-08 [SCA]
          A0FF4590BB6122EDEF6E3C542D727CC768697734
    uid           [ unknown] Alfa Test (demo key) <alfa@example.net>
    uid           [ unknown] Alpha Test (demo key) <alpha@example.net>
    uid           [ unknown] Alice (demo key)
    sub   elg1024 1999-03-08 [E]

You need some kind of pinentry program, because you may be asked for the current passphrase or an
export passphrase:

    $ cat $GNUPGHOME/gpg-agent.conf
    pinentry-program /usr/bin/pinentry-x11

Now export the key:

    $ g10/gpg --export-secret-keys alpha >/tmp/alpha.gpg

Now I create an empty home, and import the key in batch mode:

    $ export GNUPGHOME=$(mktemp -d)
    $ g10/gpg --batch --import /tmp/alpha.gpg
    gpg: keybox '/tmp/tmp.bL2caQmZri/pubring.kbx' created
    gpg: /tmp/tmp.bL2caQmZri/trustdb.gpg: trustdb created
    gpg: key 2D727CC768697734: public key "Alfa Test (demo key) <alfa@example.net>" imported
    gpg: key 2D727CC768697734: secret key imported
    gpg: Total number processed: 3
    gpg:               imported: 1
    gpg:       secret keys read: 3
    gpg:   secret keys imported: 2

Could you please check if that works for you?

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)