Xendesktop 7.6 :Unable to connect to data store.

38 The database specified in the connection string, or selected in the Options > Connection Properties tab of the SSMS connection dialog, is no longer valid or online (it might be set to AutoClose or the user may simply not have permission). I came across this once when I typed here instead of picking that option from the list. This is reported as state 27 or state 16 prior to SQL Server 2008. Note that this could also be a symptom of an orphaned login. After establishing mirroring, Availability Groups, log shipping, etc. you may have created a new login or associated a user with a login on the primary database. The database-level user information gets replayed on the secondary servers, but the login information does not. Everything will work fine - until you have a failover. In this situation, you will need to synchronize the login and user information

Follow up with Microsoft or a SQL engineer.

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Problem Cause

Log Name: Application 
Source: Citrix Broker Service 
Date: 1/6/2016 5:46:22 PM 
Event ID: 504 
Task Category: None 
Level: Warning 
Keywords: 
User: NETWORK SERVICE 
Description: 
The Citrix Broker Service failed to establish communication with the XenDesktop database. 
Please ensure that the XenDesktop database is configured and running and the database instance is reachable by this machine. 

Error details: 
Exception 'Database authentication failed' of type 'Citrix.Fma.Sdk.Dal.DALAuthenticationException'. 

SQL logs show:Logon Error: 18456, Severity: 14, State: 38. Logon Login failed for user 'domain\controllername$'. Reason: Failed to open the explicitly specified database 'databasename'. [CLIENT: IP.address]

http://sqlblog.com/blogs/aaron_bertrand/archive/2011/01/14/sql-server-v-next-denali-additional-states-for-error-18456.aspx