You can sometimes see this error in the Vault Server vlog. (Note DatabaseError [109] is a generic error. The Exception(Inner) message is more indicative.)

Error: Soap Exception ( mesg-id = xxxxxxxxxxxxxxxxxxxx )
Exception: DatabaseError [109] 
Stacktrace: 
Server stack trace: 


Exception rethrown at [0]: 
   at System.Runtime.Remoting.Proxies.RealProxy.HandleReturnMessage(IMessage reqMsg, IMessage retMsg)
   at System.Runtime.Remoting.Proxies.RealProxy.PrivateInvoke(MessageData& msgData, Int32 type)
   at Connectivity.Document.Services.FolderService.AddFile(Int64 folderId, String filename, String comment, DateTime lastWrite, FileAssociationParam[] associations, BOM bom, Stream file, FileClassification fileClass, Boolean hidden)
   at Connectivity.Web.Services.v17.DocumentService.AddFile(Int64 folderId, String fileName, String comment, DateTime lastWrite, FileAssociationParam[] associations, BOM bom, FileClassification fileClassification, Boolean hidden, ByteArray`1 fileContents)

Exception(Inner): Timeout expired.  The timeout period elapsed prior to completion of the operation or the server is not responding.
Stacktrace(Inner):    at Connectivity.Core.Database.TransactionContext.OnSqlException(SqlException e)
   at Connectivity.Core.Database.SqlAccess.ExecuteReaderInternal(SqlCommand cmd)
   at Connectivity.Core.Database.SqlAccess.ExecuteReader(CommandType commandType, String commandText, SqlParameter[] commandParameters, Int32 commandTimeout)
   at Connectivity.Core.DataAccess.OwnershipAndAccessControlCheck.ProcessReadOnly()
   at Connectivity.Core.DataAccess.DirectorExtentions.PostBuild(ArrayList builders)
   at Connectivity.Document.DataAccess.Folders.AddFile(Int64 folderId, String name, String comment, Int32 checksum, Int64 fileSize, Int64 createUserId, Nullable`1 createDate, DateTime lastWriteTime, FileAssociationParam[] associations, BOM bom, FileClassification fileClass, Boolean hidden, Boolean unmanaged)
   at Connectivity.Document.BusinessLogic.FolderBL.AddFile(Int64 folderId, String filename, String comment, Int64 createUserId, Nullable`1 createDate, DateTime lastWrite, FileAssociationParam[] associations, BOM bom, Stream file, FileClassification fileClass, Boolean hidden, Boolean unmanagedFile)
   at Connectivity.Document.BusinessLogic.FolderBL.AddFile(Int64 folderId, String filename, String comment, DateTime lastWrite, FileAssociationParam[] associations, BOM bom, Stream file, FileClassification fileClass, Boolean hidden, Boolean unmanagedFile)
   at Connectivity.Document.Services.FolderService.AddFile(Int64 folderId, String filename, String comment, DateTime lastWrite, FileAssociationParam[] associations, BOM bom, Stream file, FileClassification fileClass, Boolean hidden)
   at System.Runtime.Remoting.Messaging.StackBuilderSink._PrivateProcessMessage(IntPtr md, Object[] args, Object server, Int32 methodPtr, Boolean fExecuteInContext, Object[]& outArgs)
   at System.Runtime.Remoting.Messaging.StackBuilderSink.SyncProcessMessage(IMessage msg, Int32 methodPtr, Boolean fExecuteInContext)

It will also normally be accompanied by a Client Side User Interface error dialogue message.

This normally indicates a performance bottleneck issue that needs investigating.

It can be alleviated by increasing the SQL timeouts in the web.config file. Please refer the Advanced Configuration Guide for the version of Vault that you are using for more information.

In some situations it is necessary to limit the numbers of Users connecting to a Vault Server.

This can be achieved in a number of ways, such as using different licence servers for each Vault Server. In this blog article, I am discussing Limiting Web Site Usage by throttling the number of Concurrent Connections.

In Internet Information Services (IIS) Manager, on the Default Web Site, in the Advanced Settings, is an expandable node called "Connection Limits". there is a setting "Maximum Concurrent Connections", which in IIS 7.0, has a default setting of "4294967295". Enforcing a non-default setting requires an IISRESET.

Changing this value will limit the number of connections at anyone time to the Vault Server. When the limit is reached, a user will receive a message like this.

"The request failed with HTTP Status 503: Service Unavailable"

Notes 

Each Vault Client application will consume 1 connection. (Example 1 machine with 1x Vault Thick Client and 1x Inventor, runing and logged into Vault, will consume 2 connections)

Logged in but inactive clients release connections, which is then re-acquired following any client-server communication, such as browsing.

When mulitple users are connected to a Vault, the above error message will persist until the total number of connections to the Vault server drops below the defined limit, or the user logs into another Vault Server.

Vault Professional will poll the Vault Server every 5 minutes, see "Vault Licence Release Behaviour"

Monitoring Current Connections.

You can use the Windows Performanc Monitor to see the number of users who are connected to the Vault Server using the Web Service\ Current Connections Counter.

The instance selection will depend on if there are other websites running on the same server. For default Vault Server installations, selecting either the "_Total" or "Default Web Site" should be sufficient.