Symptoms:

   You can log onto the Windows server that has the ZCM installed using the ZCM account,
and can reach the login page but when logging-in get "User Validation Error    

Cause:

   The issue is related to the upgrade of the ZCM or the domain that was updated since the ZCM was installed    

Solution:

In the machine running the ZCM
1. open the file C:\Program Files\Zerto\ZCM\tweaks.txt
2. add a line with the tweak
t_directoryServiceValidateContextOptions=1
3. Save the file