SQL timeout expired while adding a new CRM user

I have an issue with adding new users to a 2013 on-premises, it's up to date. The error message gives no clear direction where to look:

• There are no locked processes in SQL
• Windows server log give no errors

CRM errorlog:

Unhandled Exception: System.ServiceModel.FaultException`1[[Microsoft.Xrm.Sdk.OrganizationServiceFault, Microsoft.Xrm.Sdk, Version=6.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35]]: SQL timeout expired.Detail:

<OrganizationServiceFault xmlns:i="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://schemas.microsoft.com/xrm/2011/Contracts">

  <ErrorCode>-2147204783</ErrorCode>

  <ErrorDetails xmlns:d2p1="http://schemas.datacontract.org/2004/07/System.Collections.Generic" />

  <Message>SQL timeout expired.</Message>

  <Timestamp>2016-02-12T09:03:19.8275386Z</Timestamp>

  <InnerFault i:nil="true" />

  <TraceText i:nil="true" />

</OrganizationServiceFault>

Suggestion are well appreciated!