Hello,
I have error Login failed. But after restarting the server this error doesn't appear a couple of days or weeks and everything works fine.
This problem happens only with active directory logins.
Please advise.
Thank you.

Turn on Extended debug logging in Server settings. After failing login you can look at the file log_serverDATE.txt from the log folder. It should be clear what the problem is. Most likely some AD query issue.

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Thanks.
So I open a log and had this, but it not clear for me. Please, explain:
PM Debug GetADServerInfo(1)->asit: ADHostName, suc null: True, strUserName: xxxx_admin, domain: xxar
6/21/2019 1:13:40 PM Debug GetADServerInfo(6)->asit: ADHostName)->Username: xxxxx_admin, domain: xxar, sucDomain: xxxxxhome
6/21/2019 1:13:40 PM Debug GetADServerInfo(6)->asit: ADHostName)->Username: xxxxx_admin, domain: xxar - did not find any matching user or domain
6/21/2019 1:13:40 PM Debug GetADServerInfo(1)->asit: ADHostName, suc null: False, strUserName: xxxxx_admin, domain: xxar
6/21/2019 1:13:40 PM Debug GetADServerInfo(5)->asit: ADHostName, suc null: False, strUserName: xxxxx_admin, domain: xxar - returning suc.ADHostName1: xxxxxxhome.org
6/21/2019 1:13:40 PM Debug GetADServerInfo(7)->Alternative match: xxxxxxhome.org, asit: ADHostName
6/21/2019 1:13:40 PM Debug CheckLogin->ADServer: xxxxxxxhome.org
6/21/2019 1:13:40 PM Debug GetADServerInfo(1)->asit: ADCredential, suc null: True, strUserName: xxxx_admin, domain: XXAR
6/21/2019 1:13:40 PM Debug GetADServerInfo(6)->asit: ADCredential)->Username: xxxxx_admin, domain: XXAR, sucDomain: xxxxxhome
6/21/2019 1:13:40 PM Debug GetADServerInfo(6)->asit: ADCredential)->Username: xxxxx_admin, domain: XXAR - did not find any matching user or domain
6/21/2019 1:13:40 PM Debug GetADServerInfo(1)->asit: ADCredential, suc null: False, strUserName: xxxxx_admin, domain: XXAR
6/21/2019 1:13:40 PM Debug GetADServerInfo(4)->asit: ADCredential, suc null: False, strUserName: xxxxx_admin, domain: XXAR - returning ADCredential2: af599e45-0f45-41d7-846f-143225311cec
6/21/2019 1:13:40 PM Debug GetADServerInfo(7)->Alternative match: af599e45-0f45-41d7-846f-143225311cec, asit: ADCredential
6/21/2019 1:13:40 PM Debug CheckLogin->strCredentialId: af599e45-0f45-41d7-846f-143225311cec
6/21/2019 1:13:40 PM Debug Querying for AD user: xxxxx_admin@XXAR (xxxxxxhome.org)
6/21/2019 1:13:40 PM Err CheckLogin->Unhandled error when quering for user: xxxxx_admin, domain: xxarADServer: xxxxxxhome.org, error: System.Runtime.InteropServices.COMException (0x800703FA): Illegal operation attempted on a registry key that has been marked for deletion.

at System.DirectoryServices.DirectoryEntry.Bind(Boolean throwIfFail)
at System.DirectoryServices.DirectoryEntry.Bind()
at System.DirectoryServices.DirectoryEntry.get_Name()
at VisualCronAPI.ActiveDirectoryAPI.GetUser(String strADServer, String strUserName, ADInfoClass& ADInfo, NetworkCredentialClass nc, String strCredentialId)
at VisualCronService.PermissionsAPI.CheckLogin(LoginClass l, String connId)
6/21/2019 1:13:40 PM Debug CheckLogin->UserName: xxxxx_admin, ADInfo.DC: , ADInfo.ADHostName:
6/21/2019 1:13:40 PM Debug Query AD done. User found: False
6/21/2019 1:13:40 PM Err Logon error for AD user: 'xxxxx_admin' - no such user in Active Directory.
6/21/2019 1:13:40 PM Debug GotLoginRequest(VCClient)->SID: S-1-5-21-1054543105-3894491668-1091891700-13755, AD logon:True, SessionID: 16
6/21/2019 1:13:40 PM Debug RemoveConnection->Enter
6/21/2019 1:13:40 PM Debug RemoveConnection->Connections.Remove: uuid:7573d1c3-167b-49c5-a32b-c3546558733a;id=101
6/21/2019 1:13:40 PM Debug RemoveConnection->ConnClass->Before disconnect (WCF)
6/21/2019 1:13:40 PM Debug RemoveConnection->ConnClass->After disconnect (WCF)
6/21/2019 1:13:40 PM Debug Before sending ClientConnection (Delete)
6/21/2019 1:13:40 PM Debug RemoveConnection->Connection: uuid:7573d1c3-167b-49c5-a32b-c3546558733a;id=101 was removed
6/21/2019 1:13:40 PM Debug ConnectionManager_ConnectionRemoved->Connection removed (Server Event): uuid:7573d1c3-167b-49c5-a32b-c3546558733a;id=101

6/21/2019 1:32:15 PM Debug Querying for AD user: xxxx_admin@XXAR (xxxxxxhome.org)
6/21/2019 1:32:15 PM Err CheckLogin->Unhandled error when quering for user: xxxx_admin, domain: hharADServer: xxxxxxhome.org, error: System.Runtime.InteropServices.COMException (0x800703FA): Illegal operation attempted on a registry key that has been marked for deletion.

at System.DirectoryServices.DirectoryEntry.Bind(Boolean throwIfFail)
at System.DirectoryServices.DirectoryEntry.Bind()
at System.DirectoryServices.DirectoryEntry.get_Name()
at VisualCronAPI.ActiveDirectoryAPI.GetUser(String strADServer, String strUserName, ADInfoClass& ADInfo, NetworkCredentialClass nc, String strCredentialId)
at VisualCronService.PermissionsAPI.CheckLogin(LoginClass l, String connId)

All I could find was this: https://docs.microsoft.c...-Illegal-operation-error 

Not sure if it is the logon user or AD user you use for auth in Server settings.

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Thank you.