In Citrix XenDesktop 7.x there has been a change that DNS alias isn’t allowed anymore. See article http://support.citrix.com/article/CTX137960 for more information.
The CNAME lookups have been disabled to enforce a tighter security model in XenDesktop 7.

CNAME can be re-enabled by the following registry setting:

32-bit and 64-bit:

Registry Key: HKEY_LOCAL_MACHINESOFTWARECitrixVirtualDesktopAgent
Name: UseCnameLookup
Type: REG_DWORD
Value: 1
(1 = enabled)

For a proper registration of the workplace the following registry setting has to be added to the client also:

32-bit:
Registry Key: HKEY_LOCAL_MACHINESoftwareCitrixVirtualDesktopAgent
Name: ListOfDDCs
Type: REG_SZ
Value: deliverycontrollerA deliverycontrollerB

64-bit:
Registry Key: HKEY_LOCAL_MACHINESoftwareWow6432NodeCitrixVirtualDesktopAgent
Name: ListOfDDCs
Type: REG_SZ
Value: deliverycontrollerA deliverycontrollerB

After adjustment the registration in XenDesktop works fine 😉

In total my reg-file looks like:

Windows Registry Editor Version 5.00
; Fix to register VDA on XenDesktop 7.5
; http://support.citrix.com/article/CTX137960

[HKEY_LOCAL_MACHINESOFTWARECitrixVirtualDesktopAgent]
“UseCnameLookup”=dword:00000001
“ListOfDDCs”=”deliverycontrollerA.domain.org deliverycontrollerA.domain.org”

[HKEY_LOCAL_MACHINESOFTWAREWow6432NodeCitrixVirtualDesktopAgent]
“UseCnameLookup”=dword:00000001
“ListOfDDCs”=”deliverycontrollerA.domain.org deliverycontrollerA.domain.org”

Remark:
On 64-bit environments in XenDesktop 5.6, the setting ListOfDDCs was needed in HKEY_LOCAL_MACHINESOFTWAREWow6432Node…

On 64-bit environments in XenDesktop 7.5, the setting ListOfDDCs and UseCnameLookup is only needed in HKEY_LOCAL_MACHINESOFTWARE…
The Wow6432Node supplement seems no longer to be necessary in a XenDesktop 7.5 environment. To be sure I add them under both.

Bonus:

Sometimes the logon to a XenDesktop 7.x published desktop may take long and a black screen may be visualized.
The causes are not very clear but article http://support.citrix.com/article/CTX135782 describes the steps required to modify a registry setting on a XenApp server that will help you reduce logon times. This setting is also useful in a XenDesktop environment.

HKEY_LOCAL_MACHINESOFTWAREWow6432NodeCitrixLogon
Name: DisableStatus
Type: REG_DWORD
Value: 00000001

In total my reg-file looks like:

Windows Registry Editor Version 5.00
; Fix Slow logon and black screen on XenDesktop 7.x ; http://support.citrix.com/article/CTX135782

[HKEY_LOCAL_MACHINESOFTWAREWow6432NodeCitrixLogon]
“DisableStatus”=dword:00000001