The problem was the missing TSPI_ADVANCED_USER license. Thanks for the hint

A call to RouteRegister for Device 6923 (a VDN) returns the following error - why?

Outgoing XML 6
<?xml version="1.0" encoding="utf-8"?>
<RouteRegister xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns="http://www.ecma-international.org/standards/ecma-323/csta/ed3">
<routeingDevice typeOfNumber="other" mediaClass="notKnown">6923:CM:10.90.15.102:0</routeingDevice>
<extensions>
<privateData>
<private>
<RouteRegisterPrivateData xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns="http://www.avaya.com/csta">
<switchNameList>labme1-cm</switchNameList>
</RouteRegisterPrivateData>
</private>
</privateData>
</extensions>
</RouteRegister>
-----------------------
Incoming XML 6
<?xml version="1.0" encoding="UTF-8"?>
<CSTAErrorCode xmlns="http://www.ecma-international.org/standards/ecma-323/csta/ed3">
<operation>generic</operation>
</CSTAErrorCode>

TIA
Martin