I think I've found a bug in wrapped nodes.
By default, wrapped nodes do not expose any flow variables. You can overrule this beahviour by reconfiguring the WrappedNode Output node (the 'broken' grey box). The configuration allows you to select the variables you want to expose, either by name or by a wildcard/regex. This works fine.
If the list of variables includes 'knime.workspace', it gives an error when it tries to restore its state when you re-open the workflow. This only happens if the workflow was executed before you saved it. So you run the workflow, save it, close it and then re-open it, you get the following error on the wrapped node:
ERROR LoadWorkflowRunnable Status: Error: Error loading flow variables: Invalid flow variable, invalid prefix: knime.workspace
This bug is particularly annoying if you have been working on a rather complex, time-consuming workflow, and you wish to resume your work where you left off. Or if you want to open a workflow result from the Knime server.
The workaround is to make sure knime.workspace is not in the list of exported flow variables.