There is the answer I received from D-Link support:
Dear Customer,
Your Request:
I and some my colleagues noticed the soft v3.06 and 3.07 both work badly
with an external antenna. We made experiment and downgrade AP to v3.02 and
all
seems to be OK. So we suspect a softawre bug.
kind regards,
Adam Wolf
has been answerd as follows:
thanks for this important information. we will contact our development
bureau to solve this problem in association with a new firmware.
best regards
We hope that this answers your question and that your request is now
resolved.
This information is also available through our Support Portal at
http://support.dlink.de
Regards - your D-Link Support Team
P.S. If you have further questions regarding this request,
please reply to this eMail without modifying the subject text.