Screenshot of your commandURLExtension shows /slider?value=&2$ but it needs to be either /slider?value=%2$ or /slider?value=%2$s.
I doubt that it is related to " Content = {null}".
There are two possibilites how data is being sent via http protocol. Via the URL ( /slider?value=%2$ ) and via the body of a request ( that should be the content ).
You can try to set commandURLExtension to e.g. /slider?value=128 once the OH slider is changed in the browser the value 128 should be sent to the dimmer.
onValue and offValue can be defined to switch on/off the bulp e.g. 0% vs. 100%.
The values then will be sent via the URL not via congtent.
But of course feel free to test it.
What kind of router ? Some allow network capture.
My switch allows port mirroring that means all network traffic can be mirrored to one port. That means network traffic of all hosts that are connected to any of the ports can be captured.
To capture network traffic on your OH host you can install tcpdump and then run something like:
sudo tcpdump -i eth0 host 192.168.1.11 -w /var/log/dump.pcap
This should capture traffic where host 192.168.1.11 is involved and write it to file dump.pcap.
The file needs to be copied to your PC then and can be opened with wireshark.
In case you use a different ehternet interface than eth0 that needs to be adapted in the command.
End of capturing network traffic needs to be done with CTRL+C to abort capturing.