If end users cannot open a server project, check the following possible causes:
Are Active Directory users experiencing access problems?
Is TCP/IP enabled and configured correctly on the server?
Are the Firewall rules defined correctly to allow access from outside the SQL server?
Is the MS SQL Server running in mixed mode?
Can the Citavi database not be reached from the SQL server?
Is the Citavi Database crashing and/or do you see a lot of entries with event ID 17137 ("Starting up database 'CitaviDB'") in the Windows Event Viewer?