System.Exception: Failed to retrieve document list from the specified QVS || at QMSBackendCore.Service.ServiceImpl.<>c__DisplayClass244.<GetUserDocumentList>b__243() || at SolutionGlobal.Collections.ThreadSafeCache`2.CacheItem`1.GetObject() || at SolutionGlobal.Collections.ThreadSafeCache`2.Get(TKey key, Func`1 createObjectFunc) || at QMSBackendCore.Service.ServiceImpl.GetUserDocumentList(Guid qvsID) || at QMSBackendCore.Service.ServiceImpl.Diagnostics.CacheDocumentAvailable()
Typically when I have run into this error, communication between QVS and the Root Folder/Mounted Folders has been interrupted. I believe this lends credence to your original suspicion of a DNS issue.
So everything was working fine yesterday and then all of a sudden the services are showing as disconnected? As Peter mentioned previously, the "Disconnected" error typically refers to accounts/permissions or network issues. How are the different QV services authenticated - via Windows security groups or certificates? If Windows security groups would check for password changes or permissions. If certificates, ensure that the certificate hasn't expired. If all of these are good, then investigate potential network issues. Would start with DNS as I previously mentioned.
Updated the Lef Details – just to be sure as they were showing end date of 2017-09-29
Change the Service logon user to a different domain user with the correct permissions – and restarted the services
Delete the user name and password help against LDAP against the Active Directory option, this isn’t required when the service user is a domain user and appeared to have an incorrect password.
Everything is now up and running
The issue appears to be a corrupted profile on the service user account, would recommend that you create a new Domain user to run the services as a member of local administrators and Qlikview Administrators