I have XenDesktop 7.6 and  Windows 7 VDI’s. This Error was only with one particular VDI. VDI was Registered with Controller and there were no Network Issues. (was able to launch other vdi’s from the same Hyper-V Host)

There were No errors on Delivery Controllers and Windows 7 VM’s. Was able to ping Controller’s fqdn from VDI and vice versa.

Ran XDping tool on VDI and all the checks completed without any errors.

Rebooted VDI, Restarted Citrix Desktop service and Checked the Eventlogs and it communicated with Delivery Controller successfully and Registered. But Still i wasn’t able to Launch.

Solution:

Finally, I had to Uninstall VDA agent from the VDI and Reinstall to Fix the Issue. Very curious to find out the Cause for this issue.
Never faced this issue with Xendesktop 5.6.