Server can't access its own shares / W2012R2

Hi,

have Windows 2012R2 standalone server, patched to the level what is possible with Update. When I am logged as Administrator, if I do run and then \\servername it opens up shares. However, if I do

\\servername.ad.domainname.com (DNS if fine) then it gets to the server and asks for username and password. Whatever I type in, doesn't work. So this is initial access to the server. Just to open and see shares ... Nothing to do with shares permissions.

This is all done from console of the server. This server is not part of domain, it is standalone server. It gets wierd. If I access this server from another machine, works either way fine. Logged account is local Administrator.

Anybody expeirenced this before. IPv6 is turned off but:

Microsoft Windows [Version 6.3.9600]
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C:\Users\Administrator
C:\...