The new JSON nodes are not appearing in the node repository list. According to the list of already installed nodes, org.knime.json_2.11.1.0045686.jar is installed, but not showing up in the list of nodes.
I have tried uninstalling and reinstalling the JSON nodes, and there are no error messages appearing in the knime.log.
Probably something old conflicts with the JSON nodes. One candidate is the RSyntaxtTextArea, but in my opinion that should only cause problems when the dialog opens. A more educated guess is the 2.10 XML nodes (because there are JSON to XML and XML to JSON nodes we depend on it) or any extension that uses 2.x Jackson, but not 2.4.x. Off the top of my head might be the 2.10 JavaScript nodes, probably KREST, or some other web service accessing code (maybe even the 2.10 KNIME explorer, I am not sure). Probably others have better idea what might cause this problem.
After a lot of trial and error, it seems that there was something wrong with the Text Processing nodes - org.knime.features.ext.textprocessing_2.11.1.0045686.
I could uninstall all the nodes in the KNIME Labs section of the node repository, but the Text processing nodes would still show up in the repository list after uninstalling and restarting (but not in the list of installed features).
A manual removal of the plugins and a restart finally removed these nodes.
The JSON nodes then appeared after installation, as did the other KNIME Labs nodes.