Event ID 109 when accessing Navision 3.56

We are experiencing every morning that users are not able to access Navision, without restarting the naviserver service on the server. This has to be done several times every morning before it becomes stable. According to the naviserver-service on  the server it is running, but obviously it isn't, a restart must be performed. The server is a Windows Server 2003, Standard Edition. Clients are XP with SP3. Text in the event ID as follows:

event ID 109: It is not possible to access the file below, as it is already in use (translated from danish)

Filename: <blank>

Appreciate any input on this one, thanks!