FritzBox TR064 Binding: Error when querying for Guest Wifi State


I configured the FritzBox TR064 Binding in my OH 2.4 installation like explained in the documentation. The only difference is that my OH installation cannot reach my FritzBox 7590 in the internal network (and this is intentional), so I was using its MyFritz address. When reading the documentation of AVM I noticed that - as the MyFritz port is shared with the web interface - the URL path /tr064 needs to be appended:

# services/fritzboxtr064.cfg

as for the Items I used some of the example ones:

// fritzboxtr064.items
String  fboxWanIP           "FBox WAN IP [%s]"          {fritzboxtr064="wanip"}
String  fboxName            "FBox Model [%s]"           {fritzboxtr064="modelName"}
Switch  fboxGuestWifi       "Guest Wifi"                {fritzboxtr064="wifiGuestSwitch"}

This worked flawlessly, except for the first 2 items. When it came to the WiFi switch I encountered some issues. Enabling the Guest Wifi works, but once its enabled the Switch reverts back to off. Querying the state (in the configured interval) leads to the same error as well:

10:30:28.942 [TRACE] [ding.fritzboxtr064.internal.Tr064Comm] - Raw SOAP Request to be sent to FritzBox: <SOAP-ENV:Envelope xmlns:SOAP-ENV=""><SOAP-ENV:Header/><SOAP-ENV:Body><u:GetInfo xmlns:u="urn
2019-03-17 01:10:45.858 [WARN ] [itzboxtr064.internal.SoapValueParser] - Fault received from FritzBox for item wifiGuestSwitch in SOAP response <?xml version="1.0"?><s:Envelope xmlns:s="" s:encodin
<UPnPError xmlns="urn:dslforum-org:control-1-0">
<errorDescription>Unknown Error Code</errorDescription>

Do you have any idea what the problem might be? The problem relates to the Guest Wifi only. If I removed the Guest Wifi Thing, the error disappears.

Thanks in advance