After some investigation, I found out that VectorWorks (an architecture 3D application) send a broadcast message on UDP-port 6005. As a result, the SLM service stops immediately.
Is there a way to change this port number (so Server and Client knows what port to search on)?
Can I change it in the servercfg.xml and the servers.xml? And do I have to start the service with the -up option?

Check the SLM command line options:

$ SlmServer -U
Usage: SlmServer [arguments]
Valid server startup arguments:

-c[onfig] <file> : Read server configuration data from <file>.
-m[aster] : Force server to run in Master-mode.
-s[lave] : Force server to run in Slave-mode.
-p[ort] <num> : Specify TCP port number the server should listen on.
-up[ort] <num> : Specify UDP port number the server should listen on.
-f[ront] : Run server in foreground.
-k[ill] or -stop : Kill a running server.
-l[og] <file> : Log stdout/stderr output to <file>.
-a[ppend] : Append to logfile (only useful with -l[og] option).
-maxconn <num> : Set max. connections server can handle (default: 2048).
-set var=val : Set environment variable 'var' to 'val'.
-u[sage] : Display this usage/help information message.
-h[elp] : Display this usage/help information message.
-? : Display this usage/help information message.
-v[ersion] : Display version information.
-versnr : Display version number only.


Also, refer to the -
Administrator's Guide
Document code: U9261B US
Release: Infor Solution License Manager 7.1
Publication date: February 08

A SlmServer listens on an UDP port for broadcast messages. The SlmConfig utility and the Installation Wizard send a broadcast message to that port to view a list of all running SLM servers. By default, the SLM server listens on UDP port 6005. To set a different port, you can use the -uport parameter. Infor advises you to use the default UDP port if possible, because the Installation Wizard always sends a broadcast message on UDP port 6005. You only must set a different UDP port if an SLM server is already running on that same system; in that case the default UDP port is in use.

Ok, I will try to change the registry-key
[HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Services\BCLMD]
and change ImagePath to:
"c:\Program Files\SLM\bin\SlmServer.exe" -service -up 6006 -set SLMHOME="c:\Program Files\SLM"